I've been helping to implement RSS feeds for users of Audioscrobbler, which should be fully back online soon. These feeds are in RDF format (still to be completely finalised), and show the last 10 tracks played by that user. The idea being that a) people can easily include their last-played tracks on their blog sidebar, and b) anyone can write software to take in the feeds of their friends/users with similar tastes and squirt out a combined feed that will recommend music to them. Any kind of algorithms could be run on these feeds to adapt them as people see fit, and they're all published under a Creative Commons license - as is all the Audioscrobbler data. Here's my feed (though it's probably not something you'd want to subscribe to in a conventional aggregator).