Rocketgraph - Rocketgraph is an open-source drop in replacement to Firebase, except it's much better.
Magic Playlist - Get the playlist of your dreams based on a song
Discover on Demand - Just like Spotify's Discover Weekly, but whenever you want.
Spotalike - Spotify playlist with similar songs, according to Last.fm
Muru Music - Personalised playlists within seconds.
Spotify - Map shows when two people play same song at same time