Undrtone - Social layer on top of Spotify and Rdio from Scott Vener
SoundCloud - Enjoy music & follow favourite artists
PlaylistMap - Connect with Spotify playlist curators from around the world
Spotify - Map shows when two people play same song at same time
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.