I have been using this for quite some time and love it.
Type an artist you like and it will play that artist every few songs, the rest of the songs being similar artists. If you don't like a song, skip it. You can skip for a few songs but then have to wait a bit because their music licenses prevent them from skipping more. thumbs up if you like it and thumbs down will skip it and not play songs like the one you skipped.
( dont be frightened by the link its not some crazy virus that will destroy your computer.... although the name certainly hints so"pandora's box")
www.pandora.com