FIEND.fm - "Hot or Not" for music discovery: 2 hrs of tracks in 2 mins
Parallel - Listen to music with friends over Spotify at the same time
Pacemaker - Create and share mixtapes with friends
Slackbox - Spotify playlist collaboration through Slack
Boil the Frog - Create a seamless playlist between any two artists
TuneMeet - Chat with people who are listening to the same song