Menuet OS is a useful tool to learn how a simple OS is constructed and to do various experiments. The applications can be programmed as well in assembler which is fairly easy because of the IA-32 environment which allows to pass the parameters to the kernel functions in the registers. The Kernel is a monolithic pre-emptive, real-time kernel and already includes video drivers.
Menuet OS comes under the GPL license.
wikipedia.org dumped 2003-03-17 with terodump