AC3D is a 3D mesh editor that uses Tcl/Tk for the UI (inivis.com)

I think it's the Tcl part that's more interesting. Like how Wings 3D is written in Erlang.

because it's interesting? lots of programmers on this site!

Tcl and Tk are trailing-edge tools that are out of fashion at the moment.

Nonetheless, they have some nice qualities and capabilities. And the fact that they were designed in the 1980s/1990s means that they're relatively compact and efficient (especially compared to something like Electron.) I like Tk (and Tkinter for Python) quite a bit and find that you can build some nice and easy to use interfaces with it.

I also still enjoy the retro idea (which Apple and others seem to have abandoned) that UI elements should be visible and discoverable.

Because some people think that web UIs lack basic UI development principles and are a pain to use.

Framework name makes the story interesting.