FIEND.fm - "Hot or Not" for music discovery: 2 hrs of tracks in 2 mins
Undrtone - Social layer on top of Spotify and Rdio from Scott Vener
Pacemaker - Create and share mixtapes with friends
PlaylistMap - Connect with Spotify playlist curators from around the world
Boil the Frog - Create a seamless playlist between any two artists
BeatSense - Discover great music hand-picked by real people, in a single click. FREE and in real-time!