I was killing time on the net and found one of the best websites, its called Pandora. Since I suck at typing my ideas out, I will let them tell you what it is. This is right from the FAQ section on the website.
What is Pandora?
Pandora is a music discovery service designed to help you find and enjoy music that you’ll love. It’s powered by the Music Genome Project, the most comprehensive analysis of music ever undertaken. Just tell us one of your favorite songs or artists and we’ll launch a streaming station to explore that part of the music universe.