PlaylistMap - Connect with Spotify playlist curators from around the world
Openwhyd - Collect and Share the tracks you love.
Undrtone - Social layer on top of Spotify and Rdio from Scott Vener
Zoff - Collaborate on a live music and video playlist with your friends.
BeatSense - Discover great music hand-picked by real people, in a single click. FREE and in real-time!
SoundClick - SoundClick is a music-based social community.