QuakeJS is a port of ioquake3 to JavaScript with the help of Emscripten.
This project started to demonstrate the feasibility of browser-based gaming, and what better way than with a classic game whose core gameplay revolves around the kind of responsive APIs that are available with HTML5 today.
If you'd like to read more about the development of the project, check out my blog (http://www.inolen.com/). The project is available on GitHub (https://www.github.com/inolen/quakejs).