MenuetOS

MenuetOS is a tiny OS fully developed in assembly. It features a GUI and a TCP/IP stack. It's very cool, as it come with a ton of apps and runs off a single floppy, which includes an assembler, Linux compatablity, and complete source for everything on the floppy.

A good source for a demo & links is:

http://www.geocities.com/wemakeitbig/