An interesting 3D engine in C++:

magnum: C++11/C++14 and OpenGL graphics engine

by Vladimír Vondruš

Description:

Magnum is 2D/3D graphics engine written in C++11/C++14 and modern OpenGL. Its goal is to simplify low-level graphics development and interaction with OpenGL using recent C++11/C++14 features and to abstract away platform-specific issues.