Slackbox - Spotify playlist collaboration through Slack
Festify - Festify is a free Spotify-powered app that lets your guests choose which music should be played...
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Twinjack - Stream Spotify music with anyone with just a link
Exportify - Export your playlists from Spotify
Parallel - Listen to music with friends over Spotify at the same time