DraStic

It plays in browser and is pretty cool! I recorded some quick footage and it is running full speed on most games. Sound is available now however initially when released there was no sound in the emulator. But the sound is delayed a little but it is still enjoyable.

https://www.youtube.com/watch?v=WQbQFoHsjTo

It ain't surprising, given how powerful the language is, although of course it won't cut it much for emulating more complex systems afaik.

Here are some Javascript-based emulators for systems more powerful than NES:

SNES: http://tjwei.github.io/xnes/snesWW.html
GBA: http://grantgalitz.github.io/IodineGBA/
N64: http://hulkholden.github.io/n64js/

And then there's also, a port of MESS that emulates a variety of systems, but most not generations beyond NES: http://jsmess.textfiles.com/ This one uses Emscripten (a C++ to Javascript conversion tool) to convert MESS to JS, rather than being coded straight for JS.

The N64 emulator was especially interesting, it's just too bad that the author didn't work on it longer. He had some cool information on his blog about what he was doing to improve it.

Epic lulz at Holden for doing the seemingly impossible. Too bad he didn't continue on with his work though.

Jeff Quesnelle, the guy who made nds4droid, made a n64 emulator playable in browser that is programmed in emscripten. It runs pretty good in browser but has random freezing. Also it requires the use of firefox.
If anyone wants to check it out here is the link http://jquesnelle.github.io/mupen64plus-ui-console/