GZDoom: https://steamdb.info/tech/Engine/GZDoom/
Godot: https://steamdb.info/tech/Engine/Godot/
Love: https://steamdb.info/tech/Engine/Love2D/
FNA: https://steamdb.info/tech/Engine/FNA/
GZDoom: https://steamdb.info/tech/Engine/GZDoom/
Godot: https://steamdb.info/tech/Engine/Godot/
Love: https://steamdb.info/tech/Engine/Love2D/
FNA: https://steamdb.info/tech/Engine/FNA/
Unity 38437
Unreal 10504
GameMaker 4130
RPGMaker 2700
RenPy 1973
Godot 922
So there are 4x more games made in Unity compared to the runner-up Unreal. And both of them dwarf Godot which is the only other name I recognize in that list.RPGMaker is a piece of software that lets you make top-down JRPGs generally in the style of the 2D Final Fantasy games. Because it takes that opinionated approach it's widely considered very easy to use. You're not fiddling with e.g. a physics engine when you know what genre and style of play you're doing doesn't need it.
Likewise RenPy is designed to make it simple to build Visual Novel style games, and as the name implies it uses Python, so between using an easy, popular high level language and taking an opinionated approach tailored to one genre/playstyle, it's a highly productive and approachable engine for newcomers and indie devs with limited time.
https://steamdb.info/tech/Engine/Build/
IdTech 1-4 are all also FOSS these days:
https://steamdb.info/tech/Engine/idTech1/
https://steamdb.info/tech/Engine/idTech2/
https://steamdb.info/tech/Engine/idTech2_5/
Edit: non-commercial-only, distribution only allowed via the Internet, etc.