Last.FM - Your personal music network - Personalised online radio station wow. from the dudes who brought you audioscrobbler.

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.

props to mike.