Some of my favorites (that aren't stock apps):
Now Playing - the best movie app by far (and free)
Simplify Media - stream your iTunes library over any Wi-Fi (so I have access to my 80 gig library at school)
Last.fm (in concert with the desktop app)
ScoreMobile - for NHL, NFL, CFL, etc
Wikiamo - the best Wikipedia app hands down (and it's free)
Facebook - yes... I'm on Facebook. Yes... I'm addicted. Yes... I'm ashamed to say so. Haha.
And if/when I get a MacBook the first thing I'm buying is Things (in combination with the desktop version). It looks amazing/incredibly useful for me!