Unreal Development Kit
The Unreal Development Kit (UDK) is the free edition of the 3D Unreal game engine whic has been powering AAA games since 1999. Although primarily developed for first-person shooters it has also been successfully used to develop a variety of role playing games or virtual worlds. At this point, games created by UDk may be enjoyed in Windows, OSX and iOS platforms and the engine is being actively developped.
Programming and 3d modelling knowledge are required to develop games using this platform. The programming language is UnrealScript, which is similar to C++ or Java.