UNIX is a very
powerful, easily
extensible system that provides a
solid, mostly
stable,
multitasking base with strong
networking capabilities, upon which it is relatively
easy to construct an
OS of sorts.
Windows' domination of the
PC market probably comes out of the
aggressive policies of the
Microsoft management.
However, there are a couple of OSes that are not UNIX-based:
AmigaOS and VMS are another two OSes that are pretty modern, but not so recent, and both non-UNIX.
Magenta tells me that BeOS isn't really POSIX compliant. It doesn't have a lot of critical function calls, such as select(). Also notable is that it's not multiuser.