CrystalSpace™ is a 3D graphics engine, distributed under LGPL and written in C++.
It supports a whole big load of buzzwords and obscure computer graphics terms, and has been ported to just about every platform imaginable (several flavors of *NIX with Linux included, Windows, BeOS and Macintosh included) - there was even talk about getting licence from Sony to develop a (non-free) port of it for Playstation 2, but RMS didn't like the idea - and it supports both OpenGL and DirectX.
It is highly worth checking out, if only just to get something witch which you can prove that open source stuff can be used in gaming industry too =)
Home page: http://crystal.linuxgames.com/