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?
Not entirely sure, if ReactOS would count. Its documentation has a section on WiFi and one of the screenshots on their webpage shows Firefox, but that looks to be a rather old version of Firefox, so no idea if modern versions of it work, too, well, and how well the whole WiFi business works, too.
Well, and one could argue that it’s part of the Windows family.
https://reactos.org/