I've been trying out Pandora. It's a Flash-based music player (first 10 hours are free), you enter an artist or a song that you like and it tries to find similar music for you. The data is based off the Music Genome Project which is described this way:
Together we set out to capture the essence of music at the most fundamental level. We ended up assembling literally hundreds of musical attributes or "genes" into a very large Music Genome. Taken together these genes capture the unique and magical musical identity of a song - everything from melody, harmony and rhythm, to instrumentation, orchestration, arrangement, lyrics, and of course the rich world of singing and vocal harmony. It's not about what a band looks like, or what genre they supposedly belong to, or about who buys their records - it's about what each individual song sounds like.
Pandora's interesting and while the songs do tend to resemble each other in a superficial sort of way, after an hour of listening I can't say I've found anything I like. It seems "major key tonality, mixed acoustic and electric instrumentation and extensive vamping" doesn't really get to the heart of my musical taste.