Visualify - Visualize your top artists on Spotify
Magic Playlist - Get the playlist of your dreams based on a song
SYQEL - Bring your music to life
Spotalike - Spotify playlist with similar songs, according to Last.fm
Kaleidosync - Kaleidosync is an all-in-one Spotify music visualizer that is all set to deliver a top-notch music experience with real-time support.
Playlist Machinery - Tools that help you create & organize your Spotify playlists