Momixa - Pick two songs and get the perfect mix
PlaylistMap - Connect with Spotify playlist curators from around the world
Music Suggestions Ninja - Enter an artist's name & find similar, lesser known artists
Undrtone - Social layer on top of Spotify and Rdio from Scott Vener
Boil the Frog - Create a seamless playlist between any two artists
SoundCloud Charts - The most played tracks on SoundCloud this week