The SkyOS is a hobby operating system project that was started in 1997. SkyOS is one of about three hobby operating systems that shows promise as being a solid alternative operating system for computer users wanting a second operating system to use in addition to Microsoft Windows or Linux.
The SkyOS is only able to run on the x86 architecture of Intel-based PCs, so it won't run on Macintoshes or Unix workstation hardware (SPARC, MIPS, etc.). The SkyOS, on the surface, resembles Microsoft Windows but has features--such as protected memory, symmetric multiprocessing, multitasking, and multithreading--which Windows lacks. SkyOS has a built-in shell (but is not completely POSIX-compliant) and also a built-in GUI called SkyGI.