I'm genuinely surprised Godot isn't listed on here (unless someone already mentioned it and I overlooked it.) Godot is a powerful open source engine with loads of built in tools and well written documentation. It also has it's own language, gdscript, which Python programmers well find fairly comfortable. It also allows for making a game in C# with the aid of .NET Core or Mono.
It's a good engine, and I think the devs around here would get a kick out of it. https://godotengine.org/
EDIT: And I now see that it has been mentioned before. However I'm more than happy to sing it's praises regardless.