Magic Playlist - Get the playlist of your dreams based on a song
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Spotalike - Spotify playlist with similar songs, according to Last.fm
Join My Playlist - Listen to music live together with your friends in Spotify.
Muru Music - Personalised playlists within seconds.
Spotify - Map shows when two people play same song at same time