I think everyone know's what last.fm. For people who don't know, it's a wonderful site that you should check out.
What is Last.fm?
Last.fm is a service that records what you listen to, and then presents you with an array of interesting things based upon your tastes — artists you might like, users with similar taste, personalised radio streams, charts, and much more.
Mine is last.fm/user/andriorn, what's yours?