1st 3D game engine built in C# for .NET, Visual3D Engine's predecessor
The first 3D game engine built in C# for .NET, predecessor to the Visual3D Game Engine (https://www.poweraccess.net/visual3d-game-engine), an All-in-One Game Development Tool for Next-Gen 3D Games, MMOs, Simulations and Virtual Worlds powered by Microsoft XNA.
Open-source, cross-platform 3D rendering engine for .NET and Mono. The Axiom Engine is a high-performance C# port of the powerful OGRE engine and provides full support for DirectX, OpenGL and XNA on Windows, Linux, Android, iPhone and Windows Phone.
AgateLib - Awesome game and tool engine library. A platform independent .NET library for developing games and game development tools. AgateLib is built on a driver-based model so that new platforms and rendering pipelines can be implemented easily.
A free rendered GUI library providing windowing and widgets for the easy creation of HUDs for games. It supports themes/skinning, XML-based configuration, and works with any graphics API or rendering engine including Axiom, Irrlicht, and SDL.NET.
LITS is a game development engine, which offers basic implementations of the most needed interfaces: Graphic, Network, Sound and Input. All interfaces have several (parts plattform independent) wrappers which can be choosen as needed.