Simon pointed me at Last.fm the other night. Interesting site - It looks at what you listen to, recommends new bands that you might like based on other users with similar tastes, and will even build you a custom radio stream. Here's what I'm listening to:

It's a shame that it only takes note of what you listen to whilst on-line - it'll miss out on what I listen to whilst travelling. Even more sad, it can't take advantage of the enormous historical record that my iTunes library has kept for me: 25,879 tracks played, 73 days, 14:10:16 play time. The iTunes Registry does take account of your full history - but it doesn't do recommendations.