Torque is a sophisticated 2D/3D game engine for Windows and Linux. A free trial version can be downloaded from Garage Games' web site, and the pay version comes with the full source code. If you ever wanted to design your own game, the Torque game engine (TGE) is a good starting point. TGE comes with two example templates for a racer and a FPS game. Game design with TGE requires some serious scripting and 3D design, which makes it similar to other game engines. However, TGE comes with two key features that simplify game design: a powerful, C-like scripting language, and a built-in GUI world editor. With these features, TGE offers a much lower learning curve than for example, OGRE.
Torque has a very active community, and technical questions are usually quickly resolved.