Simple 3D viewer
Pear3DEngine is a modern and modular 3D development framework
https://github.com/horde3d/Horde3D
Collections of abstractions for Pure Data
Multi-Language Wrapper Collection for the Irrlicht Engine
Game engine supporting many 3D/2D formats and graphic effects
Bruno's RAY Tracing Engine
Obsolete: use https://github.com/VMML/vmmlib