Godot Gaming Engine
Godot is an Open Source gaming engine that allows me to create games quickly and easily using the C# language. It provides the developer with a huge range of tools that allows complete focus on the game rather than on the code behind it. C# is used for scripting purposes but Godot provides its own scripting language, too.
It is an excellent platform for building games upon. It is also free to use, no matter how successful your game becomes!
There is a great community behind Godot and it is rapidly becoming one of the go-to gaming engines out there.
See the Godot wiki page for more information: HERE
Or visit their homepage: HERE