Menuet

Menuet is a developing x86 operating system completely written in assembler language. Its current distribution features a complete and elegant graphical desktop, a tiny IRC client, a text editor, various demos and games, yet it easily fits on a bootable floppy disk. It is also impressively fast.

Just to make that clear, it's not just a boot disk -- the whole operating system fits on a floppy, with room to spare.

As of 2004, Menuet is still more a unique, assembly-oriented hacking/development platform than anything else. It has very limited hardware support and very few applications (a web browser is in development). Nevertheless it is a remarkable and promising operating system. Its install and its user interface are both faster and easier than that of many Unix clones.