Slackbox - Spotify playlist collaboration through Slack
Parallel - Listen to music with friends over Spotify at the same time
Songmash - Create playlists with music that you and your friends love
aux.app - A free web-based music platform to play music from other popular online streaming services.
Join My Playlist - Listen to music live together with your friends in Spotify.
cueued - A service to demauxratize your Spotify music