The comparative benefits of HTML5 and Flash are still being argued, but thanks to a team of three Google engineers we can mark another point up to the HTML5 side. They've managed to port Quake II's game engine over to HTML5, so that you can play it completely within the browser. The end result is 30fps gaming complete with file saving and multiplayer support.
Video demo after the cut
If you're interested in trying it yourself, the team have released all of their code here. Unfortunately there's only Webkit and Chrome versions at the moment, since Firefox doesn't support some of the HTML5 code necessary.