UDK is Unreal Engine 3 – the complete professional development framework. All the tools you need to create great games, advanced visualizations and detailed 3D simulations. The best tools in the industry are in your hands.
Final latest version of the UDK by Epic, the February 2015 release. Unreal Development Kit is the free edition of Unreal Engine 3.
Epic no longer supports UDK or offers a download mirror, and recommends beginning new projects to use Unreal Engine 4, which brings all the latest engine features as well as full source code access.