Well,  here are some things I didn't know:
Mileage Plus Shopping
If you go to the Mileageplusshopping.com, you can register up to five credit cards (it doesn't have to be just a United card), then if you click through that site, you get to double dip by getting additional miles as well. For instance REI has a 2 miles per dollar. So if you spend say $500 at REI, then you get 1,000 United Miles (worth about 2.5 cents per mile or$25!) and if you use your REI credit card, you also get a 5% discount as well. Woohoo!
Also theoretically, if you use these cards while you shop at those stores in a real location, you should get credit. This only works for a small set of retailers. Non interesting as far as I see for me.
Some of the interesting stores that you can click through include (at least for me):

• REI.com. 2 miles per dollar for online orders.
• Godaddy.com. Some amazing deals here although the amounts are small, but big multipliers
• Abe's of Maine, ABT Electronics, Beach Camera, Newegg, Crutchfield, MacMall. All good electronics sellers.
• Blue Nile
• Drugstore.com
• Nordstrom.com
• eBags.com
• Golfsmith and The Golf Warehouse. Also sign up for their rebate program 🙂
• iTunes.com
• Overstock.com