OpenMW is a free and open source engine which re-implements and extends the 2002 Gamebryo engine for the open-world role-playing game The Elder Scrolls III: Morrowind. It is written in C++ and uses OpenSceneGraph for graphics and OpenAL for audio. You must own Morrowind to use OpenMW.
Facade is a first-person, futuristic, 3D, cross-platform (Windows, Mac OSX, and Linux) open-source adventure game project, built using the Ducttape Engine. Players will use nanotechnology to solve puzzles and progress through the game world.
Rigs of Rods is an open source simulator for soft-body physics
What makes Rigs of Rods different to most simulators is its unique soft-body physics simulation: Objects are simulated in real-time as flexible soft-body objects, resulting in a very realistic simulation which entirely depends on the
... [More] physical construction of the object. The community contributes greatly to the game, giving it a vast selection of vehicles and terrains from which you can choose. Rigs of Rods is mostly used for simulating vehicles, but it is flexible enough to be suited for other tasks as well. [Less]