FireNes allows you to play ~2500 Nintendo games without the need of an emulator or roms! We're not sure how legal this is and it'll be interesting to monitor how long Mozilla keeps this plug-in up. Head over to the official Firefox Add-ons page for this plugin and start playing all the Super Mario Bros games today!

Here's what you'll need to get started:
Firefox browser - If you're not using this, then you should consider switching from your current browser to Firefox:

FireNES Firefox add-on:
https://addons.mozilla.org/en-US/firefox/addon/7233 - Mozilla took the download link down, but you can grab it from here:

FireNES Firefox add-on

After you've added the FireNES FireFox addon, goto Tools > FireNes. This will open a List of games frame on the left side of your Firefox browser. Select Your game and a window with the vNES emulator will popup. Enjoy.. Here are the control:

 
Keyboard Controls:

NES Joypad "START" button = Enter
NES Joypad "SELECT" button = CTRL
NES Joypad "A" button = Use the letter "X" on your keyboard.
NES Joypad "B" button = Use the letter "Z" on your keyboard.