Here are some that I've discovered and would recommend:
Kindle - obviously if you have a Kindle it makes this one even better. Syncs right with your Kindle library.
RedLaser - Use your phone to "scan" any barcode and it will tell you what the item is, search for best online prices, and search for best local prices. Talk about taking the pain out of comparison shopping!
iPicasso - This is especially awesome if you use Picasa software for managing your digipics.
MLB.com - Any real baseball fan should have it.
DirecTV - A must have if you subscribe to DirecTV.
Opentable - Great for people who eat out a lot, or want to find a restaurant at a moment's notice and make reservations.
The Weather Channel - More powerful than the standard weather app.
ScoreCenter - Great for sports fans.