Not talking about all the Linux distros or different versions of Windows obviously. And my definition of “desktop” would be “able to connect to WiFi and launch a modern web browser”, since that would cover 90% of most people’s use cases.
I know of:
Windows
MacOS
GNU/Linux
GNU/Hurd
BSD
HaikuOS
What others are there?
Just for the record, MacOS is a descendant of FreeBSD. At least it was for quite a while.
It’s not. They just copied (literally) a few userspace tools from BSD back in the days, and rebased them to FreeBSD some time later.