I think the SkyOS developer has ceased development and is looking to on sell his project. Check out OSnews and search for stories on SkyOS. At one point it was a very promising and fast developing project.
The Wikipedia entry for MenuetOS is quite interesting and it is cool the entire OS is written in assembly. Though I have noticed the 64-bit license is proprietary. There is also KolibriOS (a fork of MenuetOS).
If you are interested in a Windows NT clone there is ReactOS.
The Hurd is another really cool alternative OS, and it now has two different distributions debian and Arch Hurd.