Well time once again to figure out how small and low power you can make something. Let's look at form factor, chip set, processor, memory, disk, in that order for a basic Haswell system. Here are the recommendations for a Linux running Haswell simple single board computer (and following some models like iafeche) for $584 each which is actually pretty remarkable given the performance level. • Motherboard. Mini-ITX. ASRock H87M-ITX.$104 from Newegg If you don't need overclocking. Need to make sure it runs Linux.
• Processor. Intel Core i5 Haswell. 4570S for lower power 65W TDP. (4570S is right now $194 from Amazon and$200 for 4570S from Newegg). The 4570 higher TDP 3.2 GHz part is $199 at Amazon and$190 for 4570 from Newegg, )
• Ram. 16GB. Corsair Vengeance LP CML16GX3M2A1600C10 $129 from Amazon or$126 from Newegg. There is a sale through August 31 for top rated G.SKILL Ripjaws X Series 16GB (2 x 8GB)  (PC3 12800) F3-1600C9D-16GXM for $119 with a coupon at Newegg. • Disk. 120GB Samsung 840.$97 for Samsung 840 Series 2.5 inch 120GB SATA III internal Solid State Drive (SSD) MZ-7TD120BW from Amazon or $110 from Newegg. Or$175 for the Mushkin Enhanced Chronos Deluxe 240GB at Newegg
• Enclosure. Antec ISK 300-150 with 150 Watt supply. $80 at Newegg. 5.25, 2 x 2.5? and half height expansion. 12.9? x 8.7? x 3.8? = 425 cu in. and just needs a better fan. • Cooler. The Antec needs a cooler lower than 65mm (hardwaresecrets.com) with the Noctua NH-9L being expensive but good or the Noctua NH-L12 also being in range but expensive.$50 from Newegg, $47 from Amazon. If you want whisper quiet then use the Noctua NH-L12 with just the 92mm fan for$64 at Amazon

Form Factor Decision: Mini-ITX
Here is the state of the art thanks to Linuxgizmos.com:

Six vendors announced embedded Linux-ready Mini-ITX single board computers (SBCs) supporting Intel’s newly announced 4th Generation “Haswell” Core i7, i5, and i3 processors. The Aaeon EMB-QM87A, BCM MX87QD and MX81H, DFI HM100-QM87 and HD100-H81D, iBase MI980, Kontron KTQ87/mITX, and Portwell WADE-8015 are equipped with Intel QM87, Q87, or H81 chipsets. Mini-ITX boards step up to Intel's 4th Generation Core · LinuxGizmos.com

Embedded Linux developers looking to tap into Intel’s 4th Generation “Haswell” Core processors can soon turn to seven new COM Express Type 6 computer-on-modules. The new COMs include models from Aaeon, Adlink, Advantech, Congatec, Kontron, Nexcom, and Portwell COM Express modules surf Intel's Haswell wave · LinuxGizmos.com\

Mini-ITX we know about and have used. We built a single board computer with it that was whipping fast the other day with a nice enclosure. It uses the traditional ATX power supply and has headers for all its connectors. So pretty easy to use.
Micro-ATX is slightly longer, has two PCIe slots and is generally less expensive but has more PCIe slots for graphics cards mainly

Micro ATX is usually cheaper than ATX, usually has 2 PCIe and some frills you'd find on an ATX depending on chipset, a mini ITX usually has 1 PCIe and very little frills, for a mini ITX it's more expensive, or the same price for less features but mobility is what you get in return Looking to build a new gaming rig, mini ITX/micro ATX? - Build - Systems.
The Gryphon Z87 is designed with an 8 phase digital power delivery system for the CPU to ensure the best performance and stability possible under any conditions. The following features are integrated into the board's design: six SATA 6Gb/s ports; an Intel I217-V GigE NIC; three PCI-Express x16 slots for dual-card support; one PCI-Express x1 slot; onboard power, reset, CMOS MemOK!, BIOS Flashback, and DirectKey buttons; and USB 2.0 and 3.0 port support. ASUS Gryphon Z87 TUF Micro-ATX Motherboard Review | PC Perspective

COM Express is a different beast and requires custom stuff, but you can build a nice mini-ITX system easily as Anandtech shows...

JJ put together a bunch of components for a mini-ITX Haswell build and took us through his build process. The motherboard itself is a Z87-I Deluxe, an upcoming mini-ITX Z87 board from ASUS. Also in the video you'll see JJ install ASUS' mini-ITX optimized GeForce GTX 670 DC Mini card. Finally, the chassis is pretty cool - it's the Lian Li PC-Q30 AnandTech | Building a mini-ITX Haswell System with ASUS

Chipset Decision: H87

The second thing to wrap your mind around is what chipset. Intel has no less than six different chipsets for Haswell!

The Z87 chipset is the most feature-rich chipset and is the only one to offer full CPU overclocking for supported (K-series) processors. This chipset can easily handle SLI/Crossfire configurations by allowing the 16 PCI-E lanes from the CPU to be divided into either a single x16 slot, dual x8 slots, or a single x8 plus two x4 slots. Z87, H87, H81, Q87, Q85, B85 - What is the difference? - Puget Custom Computers

Z87 supports Rapid Storage Technology, Smart Response Technology (otherwise known as SSD Caching), six SATA 6Gb/s ports and six USB 3.0 ports. In addition, Z87 also supports Lake Tiny which provides increased SSD power optimization features when using Smart Response Technology (SSD Caching). Finally, it supports two DIMMs per memory channel so it will be able to utilize up to four sticks of RAM. Z87, H87, H81, Q87, Q85, B85 - What is the difference? - Puget Custom Computers

The H87 chipset is very similar to Z87, but lacks a few important features including CPU overclocking. While this chipset can easily handle SLI/Crossfire configurations Z87, H87, H81, Q87, Q85, B85 - What is the difference? - Puget Custom Computers

H87 provides most of the same features as Z87 including plenty of SATA 6Gb/s and USB 3.0 ports. The only major features it lacks is CPU overclocking support and support for triple SLI/Crossfire configurations. Unfortunately, many motherboard manufactures attempt to push users to Z87 motherboards by limiting the number of ports and headers on their H87 motherboards. Because of this, Z87 motherboards are sometimes a better choice than H87 even when you do not need overclocking or triple SLI/Crossfire Z87, H87, H81, Q87, Q85, B85 - What is the difference? - Puget Custom Computers

The H81 chipset is the "budget" option and as such is very limited compared to the other consumer chipsets. It does not support SLI/Crossfire at all, has only two SATA 6Gb/s ports (plus four SATA 3Gb/s ports), and only two USB 3.0 headers. In addition, it does not support any of the major features found in the other chipsets like RST12 and Smart Response Technology. Finally, H81 only supports one DIMM per memory channel so it will be limited to a maximum of two sticks of RAM Z87, H87, H81, Q87, Q85, B85 - What is the difference? - Puget Custom Computers

Motherboard Decision: ASRock H87M-ITX

Newegg has it's Haswell mini-ITX boards conveniently listed as does Amazon Haswell Motherboards:

ASRock B85M-ITX LGA 1150 Intel B85 HDMI SATA 6Gb/s USB 3.0 Mini ITX Intel Motherboard. It is only $80, but trouble with ACPI and Linux reported, but otherwise is good for a basic PC with one PCIe (good for PCIe SSD), 2x8GB max ram, 2x USB 3.0. Newegg.com - Computer Hardware, Motherboards, Intel Motherboards, 4th-generation Core i5, i7 (LGA1150), Mini ITX ASRock H87M-ITX LGA 1150 Intel H87 HDMI SATA 6Gb/s USB 3.0 Mini ITX Intel Motherboard. Just$105. Same as above ut with 4x USB 3.0 or SATA increaes Newegg.com - Computer Hardware, Motherboards, Intel Motherboards, 4th-generation Core i5, i7 (LGA1150), Mini ITX

GIGABYTE GA-Z87N-WIFI LGA 1150 Intel Z87 HDMI SATA 6Gb/s USB 3.0 Mini ITX Intel Motherboard. Adds Wifi 802.11n and overclocking at $135. Newegg.com - Computer Hardware, Motherboards, Intel Motherboards, 4th-generation Core i5, i7 (LGA1150), Mini ITX Gigabyte GA-H87N-WIFI.$120 for the H87 version from Gigabyte.

f you drill down into the virtualization enhancements, you will see that the QEMU-KVM tag team that does KVM virtualization on x86 iron now supports the "next-generation Core processor", which refers to the future Haswell Core processors. We know this because Red Hat says it is supporting a slew of Haswell features Red Hat nips, tucks RHEL 6.4 ahead of RHEL 7 later this year • The Register

Processor Choice: Core i5 4570S ($182) or 4570 ($189)

Haswell comes in a dizzying array of choices (From Haswell (microarchitecture) - Wikipedia, the free encyclopedia). First you have to decide Core i7 vs. i5 (the i3) isn't available yet.

Quad core i5 parts do not support Hyper-threading. Not super necessary in most configurations

Then you have to pick between Vanilla, K, S, R and T suffixed parts where it seems like you want the S part is you can get it because it is slightly lower in power (2.9 vs 3.2 GHz). but the vanilla 4570 is fine.

None of the K parts support VT-d which supports virtualized IO. Good for VMs

R-series desktop processors feature Intel Iris Pro 5200 graphics (GT3e) but none are currently available.
S - Performance-optimized lifestyle by slightly lower clock speed (low power with 65 W TDP)

T - Power-optimized lifestyle which has half the cores (2 real/4 threads) (ultra low power with 35–45 W TDP)

Parts below 45xx as well as R and K parts do not support TSX, Trusted Execution Technology, or Vpro. Vpro is an IT feature, but TSX is for multithreading performance, TET is for secure computing code (probably not needed).

Price point wise, the Core i7 (47xx class) are at $300 , the Core i5 3GHz parts are at$209-227 (467x class) and the sweet spot seems to e the mid range Core i5 at (457x) class at $182-190. The S is slight cheaper ($8) and slightly slower, so it is underclocked basicallly. At that small difference, might be better to just get the 4570.
Case: Antec ISK-300-150

This is the most important next decision. There are two ways to go. A big open case. In many cases, it is actually nice to have all the components in an open rack system. Makes it easy to change and monitor and is from the gaming community but useful for servers too. For instance the Aerocool Strike Xair, but most of these appear to be obsolete. So on to a SFF (small form factor case).
Continuing from the small form factor computer work, the main thing to decide are the enclosures. We want something small as we don't need an optical drive or even a 3.5" drive. A single SSD is enough (and soon perhaps even less than that). Looking at the Mini-itx.com site for a mainstream but small mini-itx case, they sort it conveniently by volume of the case:

• Compucase 8K01. 200W PSU 70x 268x318mm. 5.97MM. Needs a low profile heat sink such as the 7106HP, 1x 3.5HDD, 1x 5.25
• Antec ISK 300-150. It is 7 liters with a 150W power supply

Lifehacker has a summary of their user votes:

• Silverstone FT03-Mini. This is small 7.4 x 15.6 x 9.3" tower. this is still pretty big, it needs an SFX (a smaller power supply) with three internal bays, one bottom fan and two expansion slots for $130. • Mini-Box M350 Universal Mini-ITX. This is tiny at 7.5 x 2.4 x 8.3" but needs an external PSU. • Fractal Design Node 304. These are simple 9.8 x 8.3 x 14.7".$90 at Newegg and uses a standard ATX PSU and fit an incredible 6 hard drives and 12" graphics cards plus tower coolers up to 165mm tall. $65 at Amazon Amazon has a list of Mini-ITX Low Profile Cases: • SilverStone Aluminum/Steel Micro ATX Media Center/HTPC Case ML03B (Black). This is the hands down favorite of the slim mini-ITX cases on amazon at 105mm H x 340mm D ( 13.4"D x 17.3"L x 4.2"H) with 3x3.5" drives.$64. Needs low profile PCI expansion cards and smaller ATX PSU with maximum dimensions of 150mm x 86mm x 140mm (the 140mm depth limits the optical drive depth if you have one). Maximum HSF heigh is 67-70mm so you need a low profile cooler. (Also see Anandtech Forum and Missing Remote). It is preinstalled for micro-ATX but has standoffs for DTX or mini-ITX
• In-Win BP644.200BL. 200 watt PSU. $44 includes a 1x low profile slot, 5.25 and 2 x 3.5 bays. 16"L x 13"W x 5.75"H • Antec Mini-ITX Case ISK110-VESA. 90 watt adapter. 10" x 5" x 18.1" Includes 2 x 2.5inch bays with a external brick, no expansion slots, optical, low profile CPU fan like the stock intel fan, says the HSF restriction is 50mm or less. Best with a 35TDP processor like the Core i3 • In-Win BP671.200BL. 200 watt PSU.$55. 5.25, 2 x 3.5, 10.4"W x 3.9"H x 12.2" D, low-profile slot. This is smaller than the BP644, but no customer reviews.

Newegg has a list of mini-ITX Desktop which shows the Antec ISK 300-150 is a good choice or the Thermaltake VL52021N2U

• Silverstone Sugo SG05BB-450-USB3.0. $114. 450 Watts SFX supply. 2 x 5.25, 3.5. It is the smallest Sugo at 10.8 liters. CPU coolers of up to 82mm height and 9 inch graphics cards. 8.74" x 6.93" x 10.97" = 664 cu in. • In Win BSQ656.DD120BL.$70 with external 120W PS
• Silverstone Sugo SG05BB-USB3.0. $107 with 300 watt SFX supply. This is ideal without a graphics card. • In Win K1.$100. 120 watt supply this includes a monitor arm
• IN Win BP644.200BL. 200 watt supply. $45. 5.25, 2 x 3.5 and low profile PCI card. Internal 200 watt TFX PSU. 12.7"H x 5.5"W x 10.8"D = 754 cu in and is 12.5 liters (so a little bigger than the Sugo SG05). • Antec ISK 300-150 with 150 Watt supply.$80. 5.25, 2 x 2.5" and half height expansion. 12.9" x 8.7" x 3.8" = 425 cu in.
• Thermaltake Element Q VL52021N2U. 200 watt SFX. $65. 5.25 and 2 x 3.5" and 13" x 8.66" x 5.12" = 576 cu in. The stock Intel fan just barely fits. Specs are These are also called Benc Computer Cases as well, like the DIYPC Alpha DB-6 which is just$40
Or if these benchs are too big, then the another option is a small mini-ITX case like the Streacom F1C which can handle a standard mini ITX and a CPU cooler of 34mm. It needs a PicoPSU and adapter and is just $60. Top rated at Newegg is the Antec ISK 300-150 with 150 Watt supply.$80. 5.25, 2 x 2.5? and half height expansion. And dimensionally at 12.9? x 8.7? x 3.8? = 425 cu in., it is very small. It supports HSFs up to 65cm and one low profile expansion card up to 9" long. It does come with a single 80mm chassis fan. It can hold a slim 5.25" drive (we don't need that), and dual internal 2.5" drive bays. It is not a super efficient power supply, less than 80% in fact, so Hardware Secrets liked the form factor but not the power supply. As an aside the ISK 310-150 has a silver front bezel. Note that if you don't need the 5.25 or the 3.5" but just want an SSD, you can actually remove the whole drive assembly and velcro your SSD onto the power supply. This gives you lots more space to put a cooler in.
Cooler: Cooler Master Hyper 212 Plus ($27 plus$10 rebate right now)

In nice benefit of the 4570S is that it can get away with a much smaller cooling solutions. In fact on Newegg, one user just used the stock Intel fan! But another guy got 46C with the Cooler Master 212 which is a $35 fan, so you don't need to spend much. Silentpcreview.com thinks it is a decent budget fan although the stock fan is noisy. The latest model is the Hyper 212 Plus for$22 and then there is the Hyper 212 EVO at $30 with a PWM fan and better cooling according Hitechlegion.com when overclocking (which we aren't doing). In their recommend list is pretty out of date, so you have to troll through their latest reviews, but they have the best reviews in the business. The main thing is to find a cooler that is relative shallow. We've used the Noctua DH-14 where height isn't and issue (or price) and the Noctua NHL-12 low profile where height is an issue and cost wasn't a big deal, but here we want something inexpensive and low profile: • Coolermaster Hyper 212 Plus was also recommended but this is a tall system (8.5/10 in quieting), but looking at mid-size heatsinks that won't cool as well but will fit in smaller cases • Scythe Samurai ZZ (Tweaktown also liked it) as a decent performer and quiet and very small, but it isn't available at Newegg or direct from Amazon either, although scythe USA lists it direct for$36. It is basically really hard to find.

For low profile the choices are that is less than 65mm (2.5")

• Noctua NH-L12 which we used in an earlier build and is indeed incredibly quiet and powerful but expensive at $65 (Amazon). It is probably a little overkill for a 65TDP part that isn't overclocking. When used with just 92mm fan is 66mm. • For really low profile at 37mm height the Noctua NH-L9i ($48 from Amazon) is small although not super efficient, but the best low fan at 66mm tall in the single 120mm fan configuration is the NH-L12 and most importantly, it doesn't interfere with the PCI-e slot that is most mini-ITX motherboards,
• Scythe Kozuti is a better choice and cheaper too at $30, but doesn't seem to be available anywhere except scytheusa.com Power Supply: Not Needed with Antec This system has modest power requirements. Without a big graphics card, it only needs 200 watts, so a fanless PSU is possible. The Seasonic X-400 (SS-400FL) Fanless is expensive at$120 at Newegg and it is Haswell ready and doesn't require an external brick and is totally silent.. although all Seasonic X-series are silent below 200 watts, these are the smallest X-series they make. If you don't mind a brick, then Mini-box.com seels the picoPSU-160-XT with a 192w adapter for $80. This is pretty expensive, so an alternative Seasonic SSR-360GP. ($60 Seasonic 360W 80PLUS Gold ATX12V Power Supply SSR-360GP from Amazon says it is pretty quiet). Hardware Secrets likes it's build quality
Memory: DDR3-1600 Corsair Vengeance LP $126 This isn't an overclocking system, so we just need reliable 2x8GB DDR3-1600 240-pin desktop memory memory with low profile spreaders because heat won't be an issue really. Newegg has a good list of these sorting by low price and looking upwards: • Kingston KVR16N11K2/16.$122. The main issue is that is has a CAS of 11.
• Corsair Vengeance LP 16GB CML16FXM2A1600C10 or CMZ16GX3M2A1600C10 CAS of 10 and $126 right now. • G.Skill Value 16GB F3-1600C11D-16GNT.$131. CAS of 11
• G.Skill Ares Series F3-1600C10D-16GAO. $135. Lots of users here with great rating, so probably the safe choice • G.Skill Ripjaws X Xeries F3-1600C9D-16GXM. These are CAS 11 at$140 and lots of users

SSD: 120GB Samsung 840 ($100) or if mSATA, use Plextor M5M 128GB ($114) or
It doesn't make much sense to do anything but an SSD these days, but the PCIe SSD for notebooks and in 2014, the SATA Express ships with teh new Intel Z97 motherboard is pretty interesting because it is way easier to mount and good for machines that aren't graphics intensive where you can use the onboard graphics adapter. Samsung is shipping these tiny things and they should be much faster since they don't use SATA, but go directly to the PCI bus. The first model is the Samsun XP941 with 1.4GBps read speeds!
The 120GB Samsung is the value leader at less than $100 for 120GB according to Tom's Hardware but becomes less competitive if it pops to say$110.
The value leader in terms of cost per bit is the SanDisk Ultra Plus. At 240GB for $170 but gets 3 out of 5 eggs at Newegg but the Mushkin Chronos Deluxe 240GB is recommended by Tom's Hardware and had a good newegg review and is cheaper at$165. The Mushkin Chronos Enhanced has toggle mode NAND and is faster for $175. However for compactness, if the motherboard has a mSATA connector, then the Plextor M5M (Anandtech likes it too) at 256GB seems a great choice and it support SATA 3.0 on that little controller. For more capacity, the SATA are still cheaper ($190 for the Mushkin vs. $219 for the mSATA m5m). The list of Haswell SFF boards shows these boards have mSATA and 802.11ac Wifi in mini-ITX form factor: • ASRock Z87E-ITX. mSATA 6Gbps and mPCIe for 802.11ac. Generall viewed as the most feature rich right now. • ASUS Z87I Deluxe. 802.11ac • ASUS RoG Maximu VI Impact. mPCIe for 802.11ac • eVGA Z97 Stinger. mSATA and mPCIe, no Wifi • Gigabyte Z87N-WiFi. mPCIe • MSI Z87I. mPCIe slot for Wifi • ASRock Z87M OC Formula in Micro ATX, no Wifi • ASUS ROG Maximux VI Gene. mPCIe and mSATA • Zotac H87 Mini ITX Wi-Fi. 2xGigabit Ethernet,$150, mPCIe with bundled 802.11n board, mSATA 6GBps. An attractive board featurewise but not available in the US.

Alternative Passive Case: Mini-Box M350 with PicoPSU 150-XT and 150W Adapter ($102) An interesting alternative to the above combination of CPU Fan, Power Supply and Case is the Streacom FC8 Evo. This is a fanless mini-ITX chassis that uses a PicoPSU brick for external power as long as you use a processor with TDP of less than 65W (eg the 4570S). It is$140 but also needs a PicoPSU power supply.
As yet another option, it the thermals are not too great, then you can use the Mini-Box.com M350 which has a built-in PicoPSU, so you can just add the $50 192 watt brick and the M350 supports the small Intel stock fan and can have two 2.5" drives in it for SSD and works with 65W or less TDP so a good partner with the 4570S. The enclosure is just$109 with a 150 watt power supply. Although with the brick, it isn't all that small really. Another nice thing is that you can mount them on a single DIN Rail with the M350-DIN-RAIL
Alternative Passive Case:Thin Mini-ITX Akasa Euler

Another option which is not quite Haswell ready is the thin mini-ITX. These are embedded focused products where the boards are less than 25mm high and you use a mobile chip drawing 35TDP or less. The Akasa Euler for instance is fanless and also thin. The motherboards are totally different, but the Intel DQ77KB runs Ivy Bridge processors and using an external brick. Testing showed it could handle a 55W TDP part which is pretty incredible and the case is just \$99.