BooTris

This super-small version of Tetris is written to fit into the 512-byte boot sector of a floppy disk. Assembly source code is included with the game, along with a program to install it onto any diskette.

The basic elements of Tetris are all here: the falling blocks can be rotated, moved and dropped to complete solid horizontal lines; the filled-up lines are cleared away to make room for more, and to prevent the pile from growing to the top of the playfield. However, the game doesn't keep score, doesn't show you the next piece, and doesn't speed up as you progress - these non-essentials have been dispensed with, making the game small enough to self-boot in half a kilobyte (without taking up any of the disk's visible space at all).