HeadRoom | Portable Listening. The folks at Headroom do such an outstanding job doing recommendations for headphones and portable equipment.
The best recommendations are the Etymotic headphones. I've gotten a bunch for folks. They are expensive, but boy do they sound amazing. The ER-6P is $99 and is very good. If you are splurging, the ER-4S is one of the five best headphones in the world at$220 (wow!). The ER-4P is a version that has a frequency response that is designed for portable players without great amplifiers. You can even get a converter cable for $50 that takes an ER-4P and converts it into a flat response ER-4S. For those of who want the ultimate. An iPod plus an ER-4S plus a portable headphone amplifier called a BitHead ($200) is the ultimate. The BitHead is 5 oz (so about as much as an iPod), but has a dedicated amplifier in it. Makes it sound audiophile grade. It can also plug into the USB connector of a computer and then you can hear incredible sound from your computer as well. Pretty cool if you have loot.
For me, I have the low-end ER-6P, but boy am I tempted to get the ER-4S, cable and BitHead (\$450 of splurge!).