JSNES is a very impressive javacript NES emulator by Ben Firshman. There’s a whole bunch of old-school classics you can play including my favorite, Contra.

I highly recommend you use Google Chrome to play JSNES. Thanks to its high performance canvas element, and a clever optimisation by Connor Dunn, it runs at full speed on modern computers. Mac builds are also available. Otherwise, it just about runs on Firefox 3.5 or Safari 4, but it's hardly playable.

Other than being only playable on Google Chrome, there’s also no sound. Still, it’s very impressive to see such creative use of HTML5.