From How-To Geek:
If you have a Nintendo DS there’s no need to limit yourself to just modern game releases. Read on as we show you how to turn your NDS into a retro-gaming marvel that plays old NES, Gameboy, and even Arcade games.
The Nintendo DS might not be the most powerful handheld gaming system on the market but it’s plenty powerful to emulate many arcade games and console systems of yesteryear. If you’re willing to invest a small amount of money and a small amount of time, you can easily turn your Nintendo DS into a veritable Swiss Army knife of gaming goodness.