It's an alternative version to the Ent/Materials gui (F3 in edit mode), all attributes of all entities (except pickups) are available :D

It also has options to customize boxes, platforms, elevators, flags, particles and bases, which didn't exist or were limited in the original...

Here's a side-by-side comparison:

The sauer's particle system is very peculiar, each attribute has a different function in each entity, which makes it a little difficult to put everything together in a menu, but I think I got a good result :)

How to install:
Move the extentgui.cfg file to your Sauerbraten root folder, ingame type /exec extentgui.cfg, or add exec extentgui.cfg to your autoexec.cfg.

The default bind is F3 in edit mode, or /genentgui (entget)

28/01/2020:
Fixed a missing variable (thanks to Point for pointing this out :p)

11/03/2020:
There was a problem that was causing the menu to get slower and slower

30/04/2020:
Everything should work as expected now

25/07/2020:
Everything should work as expected now