Here’s Super Mario brothers rewritten in JavaScript. It’s remarkably well done, even the physics closely match that of the original game. The only thing I noticed was a slight lag in Firefox 3.0.5 which I suspect might be due to the speed of the events rather than the actual game.
There have been several attempts to port the classic game to the web including this one which is fairly complete for 14kB of JavaScript and uses <canvas/>, though I think the one above is more like the original game.