Vivid Engine 2 is an open source 3D Engine coded using C# and C++.
.NET wrapper for the Irrlicht Engine
3D .NET engine for Windows, Linux, Mac
XNA based game engine completely written in c#
A cross-platform game framework based on OpenGL and GObject.
A continuation of the abandoned Tao Framework project.
Programmable X3D is a X3D implementation for .NET Framework
A modern 3D App framework using OpenCL to render via raytracing