1. Facebook - I hate myself a little for how much I use this, so much better/faster than the webapp was.
2. PageOnce - Essentially for keeping on top of my minutes with AT&T, knowing when my bill is due, etc.
3. Aurora Feint - Apple, please let them issue the update already. Unless the developers are lying, they've been trying to get it on the app store for over a week now.
4. Mobile News (AP thingy) - Sadly, this has made me more informed than ever.
5. AIM
Honorable mention -
Cube Runner - fun. I would have never thought that the lack of sound would be a feature, but it totally makes this game for me. I can listen to my iPod while playing, and while I'm on hold I can strike up a game of it as well.
Band - it's cool, although I'm not entirely sure I'd pay 10 bucks for it again.
t4Two free - you and a friend can play pong, you can also tilt it and the ball "follows" gravity or play it on a flat surface for a normal game of two person pong.
MySpace - i don't use myspace nearly as much as facebook, but that has more to do with the web versions than it does the app, which works well.
myTo-Dos - it works like its supposed to I guess. I'd rather it be a part of my calendar, but such is life.