Just started using LastFM. Yes, I know it's been around for a while, but sometimes I'm a little slow on getting started with these things.
Basically it's a big music statistics gathering site. It gathers data on every song you play, and allows you to categorize and tag each song. It then can build a tailored Internet radio station specifically geared towards your tastes. It will also recommend other similar artists and bands that you may not have heard of. You can post reviews, recommendations and any other info you want about the songs you listen to. All of the info you put into the system, is available to others using the system. So if I see that another user listens to a lot of the same music I listen to, I can browse his info to find new music. I can also read his reviews on new songs and see what he thinks of them. All in all, a pretty useful system. You can also specifically recommend things to other people. It has a lot of the usual social networking things that it seems every site has nowadays, but even if you don't participate in the social networking aspects, it can be pretty useful.