The Music Genome Project

How could one not be intrigued by a title like that?
Pandora Radio (http://www.pandora.com)

A radio station that's free, and attempts to serve you music based on your tastes, like an all-wise waitress in your favorite eatery who offers you dishes based on what you've ordered before - or didn't. Pandora is an online internet radio station in which the user can create "stations" - the starting point of which is generally a favorite song or artist. Drawing upon a database created by a team of musicologists, matches are suggested and played. The user is asked to give a "thumbs up" or "thumbs down" to the tunes played on their station, thereby refining the process, and allowing Pandora to more effectively deliver music that the listener will be interested in. The results can be very good - and the audience can discover musicians they may not have known were out there - one of the original intentions of the founder, Tim Westergren.

A station can also be created by genre, ranging from Pop to Jazz, Classical to Ambient.