Unreal Development Kit (UDK) is a free edition of Unreal Engine 3 that provides community access to the award-winning toolset like never before. This software release is available to anyone interested in using 3D game engine technology, including game developers, students, hobbyists, researchers, creators of 3D visualizations and simulations, and digital filmmakers. Anyone can start working with the industry-leading Unreal Engine 3 toolset by downloading UDK.

http://www.udk.com/