I'm pretty impressed so far with the number of free apps available. A few of the ones I've downloaded:
MotionX Poker: Poker dice on crack. One of the most addicting things I've ever played, and well worth the $4.99
AIM: Haven't used it much, but it seems fine.
Platinum Solitaire: Basic solitaire app with every basic solitaire game: solitaire, golf, pyramid, freecell, yukon, and spiders (I haven't heard of most of them). Probably not worth the money unless you're gonna play most of the games.
Pandora: Excellent free application. You enter the names of your favorite artists, pick one, and the application plays songs by them as well as songs by similar artists. Great if you're looking for some new music.
iPint: entertaining for about five seconds
SportsTap: seems like it'll be pretty useful during the season
AOL Radio: good free radio app
I'm definitely gonna try some more. These applications were a genius idea.