Actually it is called Zeta Windows by the coder Nero, and while it is already on my list for a long time now, I though it's the right moment to post this to a broader audience. Sure, it is only interesting for a small group of people who have a good ACS knowledge but still it is highly impressive what can be done inside ZDoom. So what is it actually? Zeta-Windows is a generic GUI API aimed at enabling unique implementations that are flexible and dynamic, specific to the needs of the user, fast, powerful, and simple to use. The entire Z-Windows API is written in C and compiled with GDCC in order to be interpreted by the ACS VM. While Z-Windows is capable of being a tool for the general (G)ZDoom community, independent developers looking for extensive interface design flexibility in custom environments without exhaustive production time spent in-code will benefit the most from the Z-Windows API.
Z-Windows gets its name not from Microsoft Windows, but from the X Window System (also ZDoom), which provides the basic GUI functionality for many UNIX-like operating systems. Just like X, Z-Windows provides just the basic GUI framework without mandating what the actual interface is supposed to look like. The term windows is both a GUI organizational concept and a programming concept the interpretations of which can vary dramatically. Z-Windows does deviate from X in that the functionality of Z-Windows is geared toward complete GUI management in a video game architecture and as such can mimic the appearance of an actual operating system but does not offer the wider functionality of one. Also, while GDCC is a full C compiler, functionality like reading and writing files is impossible as the ACS VM does not have that kind of access to the computer's operating system.
Wanna know more? And a video with some live action? Check the dev-thread!