Official source code of FreeCAD, a free and opensource multiplatform 3D parametric modeler.

3D Falling Sand Simulation - That might have gameplay of some sort in the future

Multithreaded particle dynamics engine using spatial hashing and quad tree collision detection to simulate forces among particles.

MTA:SA net.dll wrapper in C++

Multi Theft Auto is a game engine that incorporates an extendable network play element into a proprietary commercial single-player game.