landshade is a C++ library for quick tesselation of complex 3D data like terrain and other curves surfaces at different levels of detail on modern graphics hardware.
Linderdaum Engine is an open source purely object-oriented 3D gaming engine written in C++. It is designed to be an integrated solution for the development of interactive 3D applications, for game, industrial and scientific visualization.
Fandango, formerly and internally know as elcad. It is a python scripted 3d CAD application; the core is programmed on C++, and interface functions are exported to python to be used as building blocks for new commands.