The GEM 3D engine is a portable, free, open source, easy to use, software development kit which allows you to build high performance 3D graphics applications such as games.
The Embedded Systems 3D Game Software Development Kit, short ES 3D Game SDK, is an Open Source 3D Game Engine / SDK for embedded devices based upon OpenGL ES.
the Balls Graphics Library aims to provide all the tools you
need to develop artificial worlds, including a script-driven 3D
modeler, OpenGL-based renderer, physical simulation engine,
and realistic image synthesizer.
Quake II Weapons Facotry (THE Team fortress for Quake II) meets Quake III Arena WFA. The engine is founded on Q2 GPL code, and uses the WFA media. Visit the main website for more details.
CAVOR is an engine for building apps that involve both graphic and text/numeric data. Primarily GIS is the initial focus, but consider also CAD, CASE, project management (eg PERT charts), and others. Built-in scripting, C API, also supports Tcl and Java.