A completely browser-based, HTML5 (WebGL) First-Person Shooter game. Originally built for AngelHack 2012 in 24 hours.
Update, October 2013: I wrote a book, Game Development with Three.js, that explains in detail how to build a fuller in-browser game. Check it out if you'd like to learn more!
Update, February 2018: This code is now licensed under the GPL v3.