JSNES

Not much we need add to that headline, really, other than that you'll need Google Chrome to run this. Which is free, so quit your grumbling.

Ben Firshman's NES emulator runs in Javascript, and has been preloaded with such stonking old games as Zelda 1 and 2, Megaman, Bubble Bobble and Super Mario Bros. It runs at a crisp 30-40 frames a second.

As far as free retro packages go, there are few meatier. It even supports same keyboard multiplayer.

Play JSNES