gHacks has already covered ReactOS; a free and open source version of Windows, but there are many other operating systems out there which not many people use.

BeOS was a commercial operating system which died during the dot com crash. It was quite advanced and a number of hobbyists still maintain operating systems based on it. The biggest has to be Haiku, which is still in pre-alpha but now has a GCC4 build, so many more applications will be ported to it. BeOS, and presumably Haiku, are POSIX-compliant although not Unix-based.

MenuetOS is another personal favourite. Unlike Haiku, this is not POSIX or Unix-based. MenuetOS is free for educational use. The operating system has decent networking abilities, a web server, a web browser and a GUI. It even has webcam support! MenuetOS amazingly fits onto a standard floppy diskette despite this! This could be useful for a simple server with minimal demand.

AROS is an operating system based on AmigaOS and is now nearly complete to it. Unlike AmigaOS, AROS works on x86 computers. I haven’t tried it but Wikipedia claims there’s a basic version of the AMosaic web browser for it. The operating system is small too, being about 50MB big.

Have I missed any out?