Mir is a pretty notable example. In 2013, around the same time development on Wayland sessions was picking up for the other desktops, Canonical decided to try doing their own incompatible thing and designed their own display server system to replace X for the then-upcoming version of their Unity desktop.
Ubuntu switched from Unity 7 to Gnome 3 before Unity 8 and Mir were ready, leaving both projects abandoned by Canonical and handed off to the community. Long after it had already lost what little relevance it had, Mir eventually was converted into a Wayland compositor.
There is pheonix, and it is written fully in zig. I think it is a X11 protocol built from scratch, but will include modern features like HDR and VRR. It will also become compatible with Wayland apps along the way. I do not know the status, and I did not do much research on it.
Interesting, checked it now. Phoenix: https://git.dec05eba.com/phoenix/about/
The spec looks reasonable. I could see it replacing XWayland as a more modern and secure alternative for X11 apps not yet ported to Wayland.
There was W, which came before X but I don’t think any copies of it or even authentic screenshots of it are available on the internet.
The register published an article about this last month. Titular “frame” is addressed first, but the story also describes XLibre, wayback, arcan, and others
“Frame” is AI slop. I think it was explained that the whole or most of the project is written by Claude Code. https://github.com/isene/frame#license
How many bugs there are and for how long it’s going to be maintained, anyone can guess themselves.
Yes, NeWS (Network extensible Window System) from Sun Microsystems but it hasn’t been maintained for years and had commercial licensing fees, and DirectFB used in embedded systems.
And as others mentioned Mir from Canonical which now implements the Wayland protocol.