Openwhyd - Collect and Share the tracks you love.
Spotify - Map shows when two people play same song at same time
SoundClick - SoundClick is a music-based social community.
SoundCloud - Enjoy music & follow favourite artists
Music Smasher - Find any song. Music Smasher searches Spotify, Rdio, SoundCloud, Bandcamp, and YouTube.
Audiomack - The free SoundCloud alternative