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.
A cross-platform game framework based on OpenGL and GObject.
MaxDream is a cross-platform open source game(/simulation) framework based on OpenGL and GObject.
MaxDream aims to be compatible with any game techniques, so you can use MaxDream, without changing your OpenGL methodology.
A free rendered GUI library providing windowing and widgets for the easy creation of HUDs for games. It supports themes/skinning, XML-based configuration, and works with any graphics API or rendering engine including Axiom, Irrlicht, and SDL.NET.
A cross-platform terrain rendering engine that uses advanced techniques such as dynamic tessellation to render complex landscapes at high frame rates. Suitable for use in games, engineering, simulation, etc.
Networix is a Open Source Framework programmed in c# to allow users to script in lua, c# and GLSL in a OpenGL Environment using an object based interface.