Vivid3D is an open-source 3D engine with powerful 2D UI.
Vivid Engine 2 is an open source 3D Engine coded using C# and C++.
3D .NET engine for Windows, Linux, Mac
A cross-platform game framework based on OpenGL and GObject.
An open source skateboard games engine written in C# and .NET.
Open Source Virtual World and MMOG Development Platform
An extensible development framework for roleplay games.