Magic Playlist - Get the playlist of your dreams based on a song
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Spotalike - Spotify playlist with similar songs, according to Last.fm
SonicRoad - Discover great music faster than ever before
Spotify - Map shows when two people play same song at same time
Playlistor - Convert Apple Music playlists to Spotify and vice versa