I am surprised that ocornut/imgui [1] is not mentioned anywhere here. I used it for an admin interface in a past project. The author makes it a point to do work on the library based on donations (such as from Patreon, or PayPal). I suppose that's the main reason the project is alive and kicking today. OpenSource projects that are somehow financed - are in a much better situation than projects done on a spare time budget.

---

[1] https://github.com/ocornut/imgui