20v - Create a custom music channel based on a single song
Magic Playlist - Get the playlist of your dreams based on a song
BeatSense - Discover great music hand-picked by real people, in a single click. FREE and in real-time!
Spotalike - Spotify playlist with similar songs, according to Last.fm
Deezer - Deezer is a music streaming app created in France. It is available in 180 counties and gets 16 million users a month. 6 million of the users have paid subscriptions. Read more about Deezer.
Playlist Machinery - Tools that help you create & organize your Spotify playlists