Metal Gear is a stealth action-adventure designed by Hideo Kojima and originally released in 1987 by Konami for the MSX2. It was later ported to the NES, Commodore 64, and DOS. Not only it is the first chapter of the popular series, that would continue with Metal Gear 2: Solid Snake, but it is also considered a progenitor of the stealth game genre. The change to the 3D technology happened with Metal Gear Solid, released in 1999.
GamesNostalgia offers you the MSX version, packed with the emulator.