Partyhero allows your friends to control and queue what songs are playing through spotify!
PlayDeck is DJ Player in browser
Playlistify is a simple web app that allows you to quickly create Spotify playlists from concerts, DJ sets, radio shows, and other mixes available on setlist.fm and 1001tracklists.com.
Intersect.Ninja is a site that finds intersections between your Spotify playlists and your friends' public Spotify playlists, so you can see which songs and artists you all like!
The site is 100% open-source and your playlist data is processed completely by your browser, on your own device.