"Audioscrobbler builds a profile of your musical taste using a plugin for your media player (Winamp, iTunes, XMMS etc..). Plugins send the name of every song you play to the Audioscrobbler server, which updates your musical profile with the new song. Every person with a plugin has their own page on this site which shows their listening statistics. The system automatically matches you to people with a similar music taste, and generates personalised recommendations."
I've signed up and will have to see if it makes good recommendations.