Momixa - Pick two songs and get the perfect mix
Magic Playlist - Get the playlist of your dreams based on a song
Pacemaker - Create and share mixtapes with friends
Spotalike - Spotify playlist with similar songs, according to Last.fm
FIEND.fm - "Hot or Not" for music discovery: 2 hrs of tracks in 2 mins
Spotify - Map shows when two people play same song at same time