I've been keeping track of
www.audioscrobbler.com for a while now but the sign-ups have been down until a couple of days ago so I didn't post it.
Basically it's a plugin that keeps track of the music you're listening to as you're listening to it. It sees how much you play each song and what artists you're really into and then categorizes the music. It also shows you other people who have music like you. You can click on their usernames and check out what else they're listening to and stuff.
Anyway, I thought it was pretty cool. There are versions of the plugin available for most media players (Windows Media, iTunes, etc.).
For example, here's my profile it automatically created:
http://www.audioscrobbler.com/user/thatmariolover/