BuschnicK

I've said it before when the service was still called audioscrobbler: last.fm totally, absolutely, fantastically rocks!
It's another one of these up and coming social computing websites. You download a music player and optionally a plugin for your current music player of choice. The plugin will upload your musical choices from now on (if you allow it to) - thus creating a musical profile. From that info it generates a list of musical neighbours, a list of recommendations (people who shop on amazon should know them: "people who bought X were also interested in Y") and all kinds of custom playlists. But the real bummer is yet to come: via their free player you can listen to dynamically generated radio playlists all day long - specifically tailored to your tastes! If you don't like it you can just skip a song and go to the next. Or say I only want to listen to music similar to "Jack Johnson" (insert your favourite artist). I love it.

BTW, my profile and playlists are at: http://www.last.fm/user/BuschnicK/