I heard about turntable.fm on the Bombcast (a video game podcast I listen to). It basically breaks down like this: you enter a visualized chat room where you and others get a chance to add songs into the playlist.
There are tones of rooms dedicated to various music genres. They end up play out like an interesting mix tape from a friend. Or a stranger actually, since it’s all in beta right now and you can only access it if one of your Facebook friends have already done so.
Which leads me to its biggest problem: you can’t use it outside of the United States. Luckily, through the magic of internet proxies, I am able to use it at work. Which kind of sucks since I like it so much but won’t be able to play around with it at home.