Recently I learned that the Unreal Developement Kit (or UDK) is a free toolset for developing non-commercial games or simulations and it also comes without restrictions.
“What is UDK?” quoted from the UDK website:
“UDK is Unreal Engine 3 – a complete professional development framework. All the tools you need to create great games, advanced visualizations and detailed 3D simulations on the PC and iOS.”
Furthermore UDK is frequently updated. I find this kit much more sophisticated than Unity, because it contains more professional tools for creating scripts, materials, terrain, animations and so on, although some of these tools require a lot more learning time due to their higher complexity.