FireNes is a wicked little widget for Firefox that lets you play zillions of NES games right there in your browser. The plug-ing takes about 5 seconds to install (the site is in Spainish - click 'instalar FireNes') and then you'll have a sidebar jam packed with classic NES titles. All you need to do from there is pick the game and play. To set options and find out the keys, CTRL-click (right click for you PC losers).
Shame there's no full screen option - yet.