Avogadro libraries provide 3D rendering, visualization, and analysis
aka ogre v2 - scene-oriented, flexible 3D C++ engine
FlightGear Flight Simulator: free open-source multiplatform flight sim
An interactive viewer for three-dimensional chemical structures.
Doing 3D with Tcl
rayshade and POV for Mathematica Export + view
1st 3D game engine built in C# for .NET, Visual3D Engine's predecessor
A small Game Engine project.