Monster Studios has released the Reaction Engine, used to build the game
Chain Reaction, through GarageGames. The Reaction Engine is a hobbyist game engine designed for developing small games or creating prototypes.
The Windows based Reaction Engine's SDK is documented with over 100 cross-linked pages, and the code base is commented and integrated into the Microsoft Visual C++ environment. The Reaction Engine comes with all source code, a 3ds max exporter which converts most 3D models for use in the game engine, and several example programs.
The Reaction Engine is available at GarageGames for $59.95 for Windows and is sold with a no-royalty open license agreement. A demo of the Engine is also available.
