Undrtone - Social layer on top of Spotify and Rdio from Scott Vener
Openwhyd - Collect and Share the tracks you love.
PlaylistMap - Connect with Spotify playlist curators from around the world
SoundClick - SoundClick is a music-based social community.
BeatSense - Discover great music hand-picked by real people, in a single click. FREE and in real-time!
Music Smasher - Find any song. Music Smasher searches Spotify, Rdio, SoundCloud, Bandcamp, and YouTube.