Magic Playlist - Get the playlist of your dreams based on a song
Concertmaster - Classical music front-end for Spotify
Spotalike - Spotify playlist with similar songs, according to Last.fm
Music-Map - The Music-Map is the Tourist Map of Music, part of Gnod, the Global Network of Discovery.
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Poly-graph Hip Hop - See what hip hop's billboard top 10 sounded like