The tech wizards at Epic have created an HTML5 version of its Unreal 3 Engine tech demo Epic Citadel.
Unfortunately, it doesn't run in Chrome or Internet Explorer as the latter doesn't support the current version of WebGL. Epic noted on its FAQ that the Chrome Team is currently working on a fix.
Many will remember Epic Citadel as the tech demo that debuted on iOS to pave the way for Infinity Blade - of which it bears a striking resemblance to. The demo also comes with the Unreal Development Kit, the free edition of Unreal Engine 3.
There's not much to do in the demo besides walk around and look at stuff, but it is rather impressive for something in one's browser. Give Epic Citadel a spin on Epic's site.