Magic Playlist - Get the playlist of your dreams based on a song
PartyWith - Connecting people who love to party - anytime, anywhere.
Spotalike - Spotify playlist with similar songs, according to Last.fm
Flock Trips - Bachelor parties planned, organized, and squared away
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.