Here's a list of various operating systems that have been known to run on PCs. Some are thinly-veiled renamings of other OSes (for instance, DR-DOS was purchased by Novell, thus becoming Novell DOS, which later became Caldera OpenDOS).
CP/M-86 / MP/M-86 / Concurrent CP/M-86 / Personal CP/M-86
DR-DOS, which became Novell-DOS, which became Caldera OpenDOS
Windows NT 3x
Windows NT 4x
A GPLed OS for IA-32 systems that supports much of the "POSIX" stuff that allows it to generally run Unix-like software.
It provides a non-X client/server GUI that uses threading and is generally programmed using C++.