PlaylistMap - Connect with Spotify playlist curators from around the world
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
SoundCloud Charts - The most played tracks on SoundCloud this week
Boil the Frog - Create a seamless playlist between any two artists