Mix-tape apps seem to be the hot new thing, and while I've already talked about (and really like) Muxtape, I'm also having a lot of fun with Mixwit. My main complaint about Muxtape is that it doesn't offer any kind of widget for off-site use, but Mixwit's got that covered. It lacks some flexibility - you can't upload your own tracks, and I've had problems with a couple "disappearing" songs - but it's easy to use and draws from a pretty large selection of online music. (Update: It also kind of sucks that you can't edit song/artist info. Accuracy, people! Take pride in your copyright infringement!)
I'm still playing Grand Theft Auto IV, and as usual, I'm spending a lot of time just driving around and listening to the in-game radio (there nineteen stations in all - sixteen music and three talk-radio). Jazz station JNR is particularly strong, with tracks from John Coltrane, Miles Davis, Chet Baker, Sonny Rollins, and a few more. As a former jazz DJ, that makes me happy, although they could have used a few more songs - some Mingus or Lee Morgan or Jimmy Smith, or maybe Miles' A Tribute to Jack Johnson in its entirety.
Anyway, these are the tracks that I really get excited about, to the point that if they're playing, I'll usually sit in my fake car, double-parked outside a fake building, until they're over. It's like being sixteen again, only with a sniper rifle!