I'm probably running a little behind the curve here, but I heard about this weekend. It's a piece of software which you can download (for free as is usual in these Web 2.0 days), enter your favourite band name, press play and you get similar music which people have tagged with the band name played live over the internet for you. If you're getting bored with your iTunes collection or just want to find some new music it really is a superb mashup of social networking and music.