Snakelight!

Your goal: turn the lights off! Rotate left and right with J and L, and use the Spacebar to toggle the lights.

This is a game that runs on a PC, but not under Windows or Linux. It is a boot-sector game. The computer can directly boot the game with no OS at all!  There are plenty of ways you can run the game:

• DOSBox: "BOOT snakelight". From the command-line: dosbox -c 'boot snakelight'
• QEMU: From the command-line: qemu-system-i386 -drive format=raw,file=snakelight
• 86Box: Attach the "snakelight.img" file that has been padded to the length of a full 160K floppy disk. Configure a machine with i386SX or better; I found good results using the NCR PC916SX with video over an [ISA] CGA adaptor. Simply press ENTER when prompted after the warning about lost battery power.
• If you're feeling particularly brave, it is possible to flash a bootable USB and try on a real PC. It works on my laptop, but I cannot guarantee safety or compatibility with your system.

However you go about loading the game, you are presented with a checkered grid of lights. You want them all dark. Press any key to begin, and you will be immediately set in motion. Rotate with J/L, and toggle the lights with the Spacebar. Each toggle makes you longer: your skill at steering your snake is the hourglass that determines how long you have to finish the puzzle. If you win, the game will pause for you to revel in your victory. You can play again by pressing any key. If you lose, the game will again pause, waiting for your keypress to command a restart.

At only 512 bytes, the entire game fits easily in a QR code with no compression at all!