Heartbeat - Listen to music with your friends together, on Spotify
Magic Playlist - Get the playlist of your dreams based on a song
Partyhero - Pass your friends the aux!
Spotalike - Spotify playlist with similar songs, according to Last.fm
Vertigo - Vertigo is a web and Android application that allow you to listen to music with friends in real-time.
Playlist Machinery - Tools that help you create & organize your Spotify playlists