The new crop of Bleeding-edge browsers have some pretty robust capability with HTML5.  As any software engineer knows, the first thing you do with any new technology is try to get an id engine running on it.  If you haven’t already seen it, a few Google engineers used some of their 20% time to get pretty creative with this evolving browser technology and do just that.  Using The Google Web Toolkit, WebGL, WebSockets, and apparently a whole bunch of refactoring, they ported the Jake2 java version of the Quake II Game Engine to Javascript with excellent results.

You can grab the source here.