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?
QNX could qualify, but it’s not as easily available as most other OS.
Solaris is nearly dead for new development, but it’s still receiving updates (last release was 16 days ago) and can run GNOME and a browser.