Pandora

Pandora.com is a new "music discovery" internet radio station, and it has been the most fun I've had with music in recent memory. Apparently, its creators spent five years analyzing music to create a sort of taxonomical classifications system based on attributes like "use of modal harmonies", "laid back female vocal", "jazz influence", "use of strings", etc. This allows them to establish a similarity measure which they employ to generate recommendations. You give them the name of an artist or song you like, and they immediately play similar songs for you.

Because they analyzed such a broad range of musical artists, they are able to recommend tracks that you are likely to have never heard before. And because they are not relying on simple collaborative filtering, the recommendations tend to be more focused and accurate than other recommendation services.

In any case, I've been impressed. You get 10 free hours of music to try it out. I highly recommend it.