To go with the pure-JS [NES][nes] and [GameBoy][gb] emulators, [here’s a port of the 1991 classic Another World in plain ‘ol JavaScript][link]. Impressive. For now, it shares the same problem as the aforementioned emulators: No sound support.
[link]: http://www.megidish.net/awjs/ “Another World JS – Megidish.net”
[nes]: http://benfirshman.com/projects/jsnes/ “Nintendo emulator in Javascript”
[gb]: http://www.codebase.es/jsgb/ “GameBoy emulator in Javascript”