Magic Playlist - Get the playlist of your dreams based on a song
FIEND.fm - "Hot or Not" for music discovery: 2 hrs of tracks in 2 mins
Spotalike - Spotify playlist with similar songs, according to Last.fm
Pacemaker - Create and share mixtapes with friends
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Boil the Frog - Create a seamless playlist between any two artists