If your childhood was also spent with the Nintendo Entertainment System, then the website below will definitely get your blood pumping — vNES offers over 395 games from the Nintendo NES, including the well-known Contra (Rock 'n' Roll version of Contra), Super Mario, and more.
As long as your computer isn't too ancient (P3 or above, 192MB of RAM), you can own a free online NES. It should support all browsers (the author uses Firefox), and of course, it requires Java 1.5.0 or higher (since it's a Java-based emulator). That's about it, oh, remember to double-click the game window before playing. Apart from all the games being in English, everything is nearly perfect (check the control keys in the image).
It seems I haven't posted the website address yet — it's here: [http://vnes.thatsanderskid.com/]. Let’s go!