Here is another interesting one, a 3D game engine wrapper in C#
UrhoSharp - Xamarin
"UrhoSharp is a cross-platform high-level 3D and 2D engine that can be used to create animated 3D and 2D scenes for your applications using geometries, materials, lights and cameras."
It is a .NET binding to the Urho3D engine and allows developers to write cross platform code that can target Android, iOS, Windows and Mac with the same codebase and can render to both OpenGL and Direct3D systems.
UrhoSharp is a game engine with a lot of functionality out of the box"