That's kind of hard to answer because there are so many apps. I'd start by browsing the choices of free apps. Pandora, Bloomberg, Twitterific, Free RSS, The Weather Channel, Stanza, ESPN ScoreCenter are my free favs. Free games I like are Labyrinth 3D, Paper Toss, TapWord and Hanoi.
VideoUp (posts your 3Gs videos to Facebook)
Flixster <-- simply the best Movies related app of all
Yelp <-- probably the most useful iPhone app ever, in some ways it works better than the Google Maps app
This American Life
Words with Friends