Slackbox - Spotify playlist collaboration through Slack
BeatSense - Discover great music hand-picked by real people, in a single click. FREE and in real-time!
Partyhero - Pass your friends the aux!
üWave - üWave is a self-hosted collaborative listening platform.
Songmash - Create playlists with music that you and your friends love
musiqpad - Musiqpad is a social-music software platform for users to host their own ‘pads’ (rooms).