Android/iOS: DJ 51 uses shared music from your Spotify account and from all of your friends' accounts to build the perfect playlist for your party, study session, or any other get-together. As long as everyone's shares music through the app, DJ 51 mixes it all into a constant stream of tunes all of you will love.
To work, DJ 51 creates a "room," or a small network that you and all of your friends (as long as you all have the app) join together. Connect your Spotify accounts and start contributing music that you think fits the mood, whether it's a big party, a laid-back cocktail party, or a study session with friends. From there, the app studies where your music tastes and playlists intersect, and plays songs everyone will enjoy. The network works as long as you're all in proximity—as soon as one of you leaves, their contribution goes away, and when the group splits up and goes home, the room goes away too.
The nice thing about the app is that it's not so simple as just everyone dumping music into a communal playlist, and the app churning through it one song at a time. DJ 51 actually sifts through the music you share, finds similar artists that people who like everything you and your friends like enjoy, and plays it all for you. Best of all, it's completely free.