I have tons of apps, but these are the ones I use at least once every 1-2 months (some much more
):
Facebook
eBay
Amazon
LinkedIn
Twitterrific
Mark the Spot
Shazam
The Weather Channel
AroundMe
Scrabble
Bejeweled 2
Moviefone
Starbucks
Urbanspoon
eBuddy
Note- Mark the Spot is an app published by AT&T. If you drop a call or have an area that frequently has crappy coverage, you can use this app to mark it and send the data back to AT&T. A friend who works there recommends it to everyone she knows- more data = better fixes = better coverage.
Both of the games- Scrabble & Bejeweled- are paid games... but worth it.