MusicBrainz is a great start. The idea is to uniquely mark (I would use the term "fingerprint" but ...) individual recordings. Let that sink in and think about what you could do if you could unambiguously say that you had just listened to x, or that you liked x. First off, it makes a project like MoodMusic easier and better (especially the collaborative filtering part), but also it makes Fisher's plan easier to implement. And, a lot more. [via Archipelago].