I have had an account with last.fm for a while now and have always liked the service.
I just said goodbye to them.
Last.fm is a music tracking service. After you create an account, you can install a scrobbler. This is a small program that tells their site what music you are listening to as you listen to it. They keep track of what you like, and can recommend music that you may like based on what you have already listened to. They also have streaming radio stations of music that you may like, or that your friends are listening to. In short, they let you find stuff to listen to that you otherwise might not have heard of.