Pandora.com is one of the best sites I have found in a long, long time. It runs off the Music Genome Project. Here is what that is:
On January 6, 2000 a group of musicians and music-loving technologists came together with the idea of creating the most comprehensive analysis of music ever.
Pandora is the music player they created to pull songs that it thinks you will like. The play list are based off of songs and artist you told it you like. It creates an Internet radio station that’s just for you.