BeatSense - Discover great music hand-picked by real people, in a single click. FREE and in real-time!
Magic Playlist - Get the playlist of your dreams based on a song
20v - Create a custom music channel based on a single song
Spotalike - Spotify playlist with similar songs, according to Last.fm
The Midi Shrine - A collection of MIDI files for SNES, NES, GameCube, Game Boy, and others.
Playlist Machinery - Tools that help you create & organize your Spotify playlists