aka ogre v2 - scene-oriented, flexible 3D C++ engine
FlightGear Flight Simulator: free open-source multiplatform flight sim
Xj3D is an open-source Java codebase for the X3D Graphics standard.
A public domain cross platform 3D game
https://github.com/horde3d/Horde3D
1st 3D game engine built in C# for .NET, Visual3D Engine's predecessor