This is really, really, really awesome. Pandora is a radio-station front end to the Music Genome Project, which is an enormous detailed music profiling tool. Basically you give it one or more songs or artists that you like, and it begins playing music that it thinks you might like. You can thumbs-up/thumbs-down a song or skip to another, etc. The cool thing is that it does not use any "usage pattern matching" or "collective popularity" kinds of algorithms, it uses around 400 distinct musical properties that a team of musicians has analyzed for each song. So it doesn't just recommend popular music that other people bought, but instead it recommends actually similar music, based on the music itself. Very cool!
I'm gonna go broke on iTunes now.