Crystal Space is a free, cross-platform software development kit for realtime 3D graphics.
It supports Windows, Linux, MaxOS/X and more. Though written in C++, other languages like Python can also be used to write programs.
Based on a plugin system it's very modular and extended easily.
3D rendering is done with OpenGL. Features of the latest as well as older hardware are supported seamlessly.
At one point, Open Hub analyzed source code for this project based on code location(s) available at that time. Since then, the code locations have been removed.