Slackbox - Spotify playlist collaboration through Slack
Heartbeat - Listen to music with your friends together, on Spotify
Partyhero - Pass your friends the aux!
Festify - Festify is a free Spotify-powered app that lets your guests choose which music should be played...
Songmash - Create playlists with music that you and your friends love
BeatSense - Discover great music hand-picked by real people, in a single click. FREE and in real-time!