CryENGINE 3 SDK is a game development application, and the technology behind Crytek's fantastic Crysis 2 game. It is being offered totally free for non-commercial and educational purposes.
This is no casual development kit, but it's very powerful. There is a lively community of developer forums that are great for newcomers, full of helpful advice and examples of work. To use CryENGINE 3 SDK, after downloading you must register with Crydev.net, then you are totally free to get creating.
CryENGINE 3 SDK is a powerful development tool, for creating professional 3D environments and games.