Unreal Development Kit is described as '"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." -Site' and is an app in the Development category. There are more than 25 alternatives to Unreal Development Kit for a variety of platforms, including Windows, Linux, Mac, Online / Web-based and Android. The best alternative is Godot Engine, which is both free and Open Source. Other great apps like Unreal Development Kit are Unity (Free Personal), Unreal Engine (Free), GDevelop (Free, Open Source) and CRYENGINE (Free Personal).
Godot is a fully featured, open source, MIT licensed game engine. It focuses on having great tools, and a visual oriented workflow that can export to PC, Mobile and Web platforms. It is a cross-platform 2D and 3D game engine and game development IDE.
Unreal Engine is a professional suite of tools and technologies used for building high-quality games across a range of platforms. Unreal Engine’s rendering architecture enables developers to achieve stunning visuals and also scale elegantly to lower-end systems.
GDevelop is an open-source game making software designed to be used by everyone. Any kind of 2D games can be created with it and no programming skills are required to get started. Logic of games are made using visual events.