Opening Pandora's Box. Literally.
Go to pandora.com, put in your favorite artist, and up pops a custom-designed radio station featuring that artist, and similar artists. If a song comes up that you don't like, you can click a button to skip it. If you want to purchase a track you hear, you can click a button to take you to Itunes or Amazon, and you're in business.
It's powered by the Music Genome Project, which you can read about on the site.
Another one of those cool websites you wish you'd thought of, but didn't.
Oh well. At least we can enjoy it now that it's here.