4K of Java code may not pack in the visceral thrill of its DVD-sized predecessor, but Markus Persson's Left 4k Dead is still impressive as a teeming mass of efficiency.
Built for the annual Java 4K coding competition, Left 4k Dead makes sure to capture the essence of Valve's zombie apocalypse shooter, but does without the fancy graphics, moody soundtrack and four-player co-op play. It's just aim, shoot, kill, survive.
If you've got a mouse, a mouse button and the necessary keyboard keys, you meet the hardware requirements. Give it a whirl.
Left 4K Dead [Mojang - thanks, Kevin, et al!]