Replayify - Create playlists from your most played Spotify tracks
Slackbox - Spotify playlist collaboration through Slack
Spotify - Map shows when two people play same song at same time
Partyhero - Pass your friends the aux!
CoverSutra - CoverSutra gives you a handy and attractive way to control iTunes without ever leaving your current...
Songmash - Create playlists with music that you and your friends love