Avogadro libraries provide 3D rendering, visualization, and analysis
aka ogre v2 - scene-oriented, flexible 3D C++ engine
LDraw™ editor for LEGO® style digital building instructions.
Realtime raytracer using SIMD on ARM, MIPS, PPC and x86
Vivid is a modern C++ 3D engine using OpenGL4+
Vivid Engine 2 is an open source 3D Engine coded using C# and C++.
Program for drawing 3D surfaces from parametric equations
shader editor
https://github.com/horde3d/Horde3D
Bruno's RAY Tracing Engine