The Massively Multiplayer Piano
What is it?
It's a game! You gather in rooms, and the game challenges you to play chords. It gets harder as time goes on.
How is it?
It's built using Node.js, in CoffeeScript. It also uses Midi.js, which is a huge thing to pull in but it was super-easy to use.
The MMP was built in less than 24 hours for Music Hack Day, while running on far too much caffeine and sugar. It's probably not very good code. This is the first ever Node.js app I've ever built. I didn't really read any tutorials beforehand. I knew it was event-driven and I'd seen a few bits of example code; that was enough for me to muddle through.