I recently became an major fan of and addicted to Pandora Radio. Based upon the Music Genome Project, Pandora uses a large database of songs to find songs and artists similar to a song (Or multiple songs or artists.) that you like. The great thing is that it's completely free, and with AdBlock Plus the unsightly ads on their site are no more. Though their UI could use some work in my opinion, Pandora Radio will remain on my bookmarks for years to come.