I've been playing around with Audio Scrobbler for the past couple of weeks. If you're not familiar the premise is that you allow the website to monitor the music you listen to on your computer (through a variety of plug-ins). Based on this data the website can do various things; create charts of your favourite artists and tracks, recommend new music based on the familiar "members who liked this also liked..." scenario, generate a personal radio station and also introduce you to like-minded music fans.
The site is a bit rough around the edges just now, but in six months time I could see this being to music what Flickr is to pictures. Right now however It's just keeping me amused in two ways, first it can show me that I listen to a lot more of The Human League than I realised, secondly it allows me to keep an eye on what my friends are listening to, and I guess they can watch what I'm listening to in real-time too.