Twinjack - Stream Spotify music with anyone with just a link
Slackbox - Spotify playlist collaboration through Slack
BeatSense - Discover great music hand-picked by real people, in a single click. FREE and in real-time!
Playlist Machinery - Tools that help you create & organize your Spotify playlists
ReverbNation Discover - Stream hand-picked, ad free music from emerging artists.
Jelly Party - Watch anything together.