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
Vivid3D is a C++ IDE/3D engine, with python scripting.
Doing 3D with Tcl
Vivid Engine 2 is an open source 3D Engine coded using C# and C++.
sauerbraten with LuaJit2 scripting