Quesa is a 3D rendering and scene graph library for macOS and Windows, with a BSD license. It was begun way back in the year 2000 as a re-implementation of Apple's QuickDraw 3D APIs, using OpenGL 2. One can still get a pretty good understanding of how it works, such as the use of reference counted objects and the 3DMF object file format, in Apple's QuickDraw 3D manual.
The Quesa project was begun by Dair Grant. At its period of maximum activity, it was also administered by José Cruanyes, Roger Holmes, Kevin Matthews, Joe Strout, and James W. Walker, with contributions from a few others. Now it is solely maintained by James W. Walker.
In the near term, I expect to add support for rendering with OpenGL 3.2 Core Profile. I also expect to contribute code to import and export 3D models is Collada (.dae) format.
In more of a "hopes and dreams" category, it would be nice to support other rendering APIs such as Vulkan or Metal.