TuneIn Radio is good, if you like radio. Redlaser is quite useful, uses the camera to scan barcodes and then search for prices on the item.
Pocket God is fun, harass little natives. Plants vs Zombies, Peggle and Knights are quite good, only games I've really tried to finish on it..
Ummm.. Soundhound is pretty cool, they have a free version too.. lets you find out what music is playing, even if you're just humming it.
Shazam is similar.
There's a WoW armory one, but I suppose you'd need an active WoW account for it.. also fishing and auction house too, I believe.
My 3G can't do it, but I hear the augmented reality apps can be useful.. those are the ones where you look through the camera and see virtual signposts or such, telling you where everything is in real time.. like a map, but your standing in it. Can do other stuff too, like paste a nude body under a person's head on the cam.. if you're into that sort of thing. Lol..