You chose a song or artist and it plays stuff that it thinks sounds similar. So you get to hear loads of new music that you might not have thought about for free. It's like your own personalised web radio station. I've been listening to "lemonjelly radio" and "aim radio" lately..
Massive network of personal webspace with room to upload songs, photo's bloggs.. you get messages, instant messages etc. Particularly useful for musicians and artist's I think.
Funny ( and slightly sick ) cartoons, usually revolving around a gerbil struggling to stay alive.