Magic Playlist - Get the playlist of your dreams based on a song
JamBuddy - Find and collaborate with musicians around you thru Spotify
Spotalike - Spotify playlist with similar songs, according to Last.fm
Stereofox - Find awesome new music, explore artists & create playlists
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Spotify - Map shows when two people play same song at same time