Playlist Machinery - Tools that help you create & organize your Spotify playlists
Magic Playlist - Get the playlist of your dreams based on a song
Playmoss - Create playlists from YouTube, Vimeo and SoundCloud
Spotalike - Spotify playlist with similar songs, according to Last.fm
podsync - Simple and free service that lets you listen to any YouTube or Vimeo channels, playlists or user videos in podcast format
Spotify - Map shows when two people play same song at same time