Magic Playlist - Get the playlist of your dreams based on a song
Rocketgraph - Rocketgraph is an open-source drop in replacement to Firebase, except it's much better.
Spotalike - Spotify playlist with similar songs, according to Last.fm
Plug - Lightweight OSX player for Hype Machine
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Muru Music - Personalised playlists within seconds.