Audioscrobbler / Last.FM

If you haven't seen Audioscrobbler yet, you might want to look at it. It's quite an interesting service...

"Audioscrobbler builds a profile of your musical taste using a plugin for your media player (Winamp, iTunes, XMMS etc..). Plugins send the name of every song you play to the Audioscrobbler server, which updates your musical profile with the new song. Every person with a plugin has their own page on this site which shows their listening statistics. The system automatically matches you to people with a similar music taste, and generates personalised recommendations."

And Last.FM "is a personalised online radio station that plays the right music to the right people. Songs spread from listener to listener.
You get your own online radio station that you can fill up with the music you like. This information is used to find users who are similar to you. With this information Last.fm can play you new artists and songs you might like."