WebSounds/piano is an interactive, collaborative music making machine.
You can see the entire project source on GitHub. It was designed by John Mars for Experimental Data Representation, a course at RISD in Spring 2014.
How to play
Use your keyboard or mouse to hit the notes, and play along with everyone else. You can see who's playing what by looking at the piano roll along the bottom, and paying attention to the lights up top.
Notes gradually turn red the more you play them, and the background gets lighter the more people are playing.
Chat with the other musicians in the window to the left. Work together to create something amazing.