Pandora is a website that runs what they call the Music Genome Project. You enter a song, artist or genre, and they make a free radio station based on your choice.
For example, I type in Type O Negative. They play a song typical of TON's style, and I get to give it a thumbs up or thumbs down. Depending on my choices, the program then selects another song that it thinks I ought to like... doesn't ALWAYS work out, but more often than not it does.
The only problems are that if you dont like a song, there are only so many songs you can skip in an hour, and Pandora is just a program- it's only as good as you are sure of what you like.