Now, this isn't as music-related as it is awesome, but it's related to using your computer to jam out rhythms and melodies, so we'll roll with it.
This is Jam With Chrome, just one of many Chrome Experiments, which are described here—and excerpted below so you can get straight to the jammin':
Experiments is a showcase for creative web experiments, the vast majority of which are built with the latest open technologies, including HTML5, Canvas, SVG, and WebGL. All of them were made and submitted by talented artists and programmers from around the world.
We hope the projects on this site provide inspiration for anyone interested in experimenting creatively with the web. And we hope they show how the web has become faster, more fun, and more open — the same spirit in which we built Google Chrome.
Seriously, this is fun. I spent about five to ten minutes putting shame to my surname and genetic lineage by proving that I don't have great rhythm (something which a few videos that exist somewhere could prove, I imagine), but I had a great time doing it.
Give it a few minutes of your life. You'll enjoy it. And, if I'm understanding it right, it looks as if you can invite friends to join the same session and "play" instruments with you, thereby multiplying the enjoyment. Everyone wins!