Jad Engine is a Managed DirectX 3D Game Engine, made with C# 2.0 and opensource.

It is quite simple to use, yet powerful. Features things like a complete AI system, Newton Physics, lots of 3D effects, advanced texture & lighting techniques, adn exporter for 3D models from 3D Studio Max.

In development are a scripting system, a full scene editor (separate project), and lots of other stuff.