Festify - Festify is a free Spotify-powered app that lets your guests choose which music should be played...
Magic Playlist - Get the playlist of your dreams based on a song
Twinjack - Stream Spotify music with anyone with just a link
Spotalike - Spotify playlist with similar songs, according to Last.fm
Parallel - Listen to music with friends over Spotify at the same time
Playlist Machinery - Tools that help you create & organize your Spotify playlists