FreeDOS

No more rummaging around the shoeboxes for that old DOS 6.2 floppy to boot up a freshly formatted hard drive: FreeDOS aims to be a complete, free, 100% MS-DOS compatible operating system.

“People have … been re-writing basic OS code for so long that all of the technology that constituted an “operating system” in the traditional (pre-GUI) sense of that phrase is now so cheap and common that it’s literally free. Not only could Gates and Allen not sell MS-DOS today, they could not even give it away, because much more powerful OSes are already being given away.” – Neal Stephenson, “In the Beginning was the Command Line”

Heh heh.