The Irrlicht Engine is an open source high performance realtime 3D engine written and usable in C++. It is completely cross-platform, using D3D, OpenGL and its own software renderers.
XImaging is a lightweight, multi-threaded image browser and viewer for Unix - X/Motif, with basic file management functionality.
Documentation and more on the project homepage: http://fastestcode.org/ximaging.html
OpenArena is a 3d game creation engine. It is a rendering system that can be used for more than just games but it is targeted at games. The basic use being create some game media textures models maps and config files for physics and rules.