MAME

MAME is a free and open-source emulator designed to emulate the hardware of arcade games, later expanded to include video game consoles, old computers and other systems in software on modern personal computers and other platforms. Its intention is to preserve gaming history by preventing vintage video games from being lost or forgotten. It does this by emulating the inner workings of the emulated machines; the ability to actually play the video games is considered "a nice side effect". Joystiq has listed MAME as an application that every Windows and Mac gamer should have.