The TuxCap Games Framework is a GNU/Linux and Mac OSX port of the PopCap Games Framework used for 2D game development. It comes with PyCap Python bindings, a fast 2D physics engine, a particle engine, widgets and many documented examples.
Objective-3D is a video-game library developped by Dream Overflow. It works on PC and Machintosh, and it is developped for Windows, Linux and MacOSX plateforms. Developed in C++.
Yet Another Homemade Game Engine (yahge) is what the name says a free open source 3D Game Engine geared towards development of 3D First Person Shooters. Major goals are to provide a Rapid Prototyping System for Game Ideas or to make Game Demos for Resume