Shared Spotify - Find the common songs you have between friends
Parallel - Listen to music with friends over Spotify at the same time
Spotify Codes - Share codes for songs, artists, and playlists from Spotify
Slackbox - Spotify playlist collaboration through Slack
Bands on a Budget - Economical SWAG meant for bands, often hijacked by startups
Join My Playlist - Listen to music live together with your friends in Spotify.