Rocketgraph - Rocketgraph is an open-source drop in replacement to Firebase, except it's much better.
Spotify - Map shows when two people play same song at same time
Plug - Lightweight OSX player for Hype Machine
Muru Music - Personalised playlists within seconds.
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Magic Playlist - Get the playlist of your dreams based on a song