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?
Serenity was getting close, but I think that whole operating system took a bit of a back seat ride when they started with the gigantic task that a modern browser is. Hence Ladybird.