If you live in one of the big cities and don't have a restaurant app that you love, I just tried out this one called MenuPages (free) and you look up the area you're in and it shows every restaurant in a Google maps style layout that you can click on for reviews, directions and the full menu. I think it's a newer app so they're still adding places by the day but they already have a bunch in there.
I also got a lot of medical flashcards on the phone, so thats helping me study as well.
Some other random things:
TV Guide - Cause I'm just too lazy to hit the menu button and find whats coming on, it has a nice search feature so I know when to turn the boob tube on.
Pdanet - Turns the iPhone into a wireless tether.
Broadcaster - Stream live from your phone.
Unblock Me - Just a time killer and to keep the brain moving
gPush - Very strongly recommended for gmail users. Puts up a push notification screen every time I get a new msg.
iRwego - Okay so it plays the Mario theme, but when you jump it makes the Mario jump noise. This is pretty fun to show off at parties.