IMO an absolute must-have is vindigo. if you live in a big city, it has detailed info on restaurants (actually it has the Zagat guide), customized to your address, with walking dirctions, subway directions, etc. I live in Boston and Vindigo is an absolute lifesaver. It also gives movie listings with show times, and shopping info too.
Just don't let anyone beam it to you because that might cause your palm to crash. also, be careful installing it with avantgo, as the two did not get along well on the same palm until recently. before installing vindigo with avantgo, make sure to check their website thoroughly.
avantgo is another good application, though I wouldn't quite consider it killer.
AG