Firefox only (Windows/Mac/Linux): The FireNes Firefox extension integrates over 2000 old-school NES games with your favourite browser. Once you install FireNes and fire it up (either through the Tools menu or the optional toolbar button), you can browse the enormous library of games in your Firefox sidebar and play any one of your 8-bit favourites at the click of your mouse.
Alternately, if you'd prefer not to install another extension, you can get similar goods add-on-free at web sites like previously mentioned vNES or Nintendo8. Either way, use wisely; this is a bit of weekend fun, but let's not get carried away when the time to get things done rolls back around. FireNes is free, Firefox only, requires Java.