Crystal Space is an Open Source 3D SDK for Unix, Windows, and MacOS/X. It renders with OpenGL and features GLSL shaders, CG shaders, deferred rendering, dynamic shadows, bullet based physics library, terrain engine, skeleton based animation meshes, exporter for Blender, portals, etc...
C-Lesh is a game programming language for 2D games.
C-Lesh is a game programming language for 2D games - mainly arcade and platform games. When combined with the Super Mario World (SMW) engine you can create awesome 2D platform games like Super Mario World. This language evolved from a "clone" of TI-BASIC which I began developing when I was having memory (segmentation fault) bugs with C. C-Lesh lacks memory allocation and destruction. There is no need to use pointers and you can never get memory leaks or program crashes caused by memory...
World Foundry is a 3D level gameengine and asset production pathway with over 170,000 lines of code. Originally a commercial product, now released under the GPL. Designed from the beginning to be cross-platform, runs on windows, working on linux port.