The frontend was stolen from DayDun and the backend re-implemented from scratch. Rooms and rate limit functionality was stripped out, each connection is a different user, and raw IP addresses are used for user ids.

The notable thing about this and DayDun's piano is that note events are simply, individually, immediately broadcasted in a minimal binary format, and clients instantly play notes they receive; unlike Brandon Lockaby's Multiplayer Piano which buffers note events with timing data into JSON that's sent every 200ms and then played exactly one second after they actually happened. (PianoRhythm does something similar.) Doing this preserves the exact note timing regardless of networking quality (unless it takes longer than a second for the data to get through), which is fine for listening to other players, but a problem when playing together. With this piano, notes go directly through as fast as possible, which is perfect for local networks, and reveals the true networking quality and latency over the internet.

It's also super simple to run if you do want to use it on your local network; just download, npm install, node server.js and connect to it (default port is 924).