Vivid3D is an open-source 3D engine with powerful 2D UI.
2D Game Engine
Modern 3D engine and IDE written using C# and C++.
Portable game engine with main focus on point-and-click adventures
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.