Shared Spotify - Find the common songs you have between friends
Magic Playlist - Get the playlist of your dreams based on a song
Playlist Souffle - Swap Spotify playlist tracks based on album or artist
Spotalike - Spotify playlist with similar songs, according to Last.fm
Gigstape - A mixtape of concerts you'll ❤ in your city this week
Playlist Machinery - Tools that help you create & organize your Spotify playlists