It's called Pandora and was created by the Music Genome Project. You add a singer, band or song that you like, and it'll search for similar artists/songs. While you're listening to the song, you can vote if you like it or not, and depending on what you choose, the songs played will change to fit your preference. It's kind of hard to explain, but you should get it a try. It's a good find to find new artists that are similar in taste to the ones you already like. It even tells you why they think a certain band or song is similar to the one you entered.
Pandora
