I have read some news about SkyOS and it seems to me very interesting. What do you think about it? I'd like to test it but it's not free for downloading (you have to pay $30 for receive a beta version on CD).
SkyOS is an operating system written from scratch for x86 PC's. As required for a modern operating system, it supports features such as multiprocessor support (SMP), virtual memory, memory protection, multitasking and threading etc. Additionally, SkyOS has a built-in GUI system named SkyGI.
For more informations website
and an article