Unreal Engine is Epic Games' game engine, a powerful suite of tools for developing anything from basic 2D games to professional RPGs, first person shooters and whatever else you like.
There's a vast amount to learn, of course, and that's even before you start building your game. But there's plenty of documentation, tutorials, demos and sample projects to point you in the right direction.
The package is now entirely free, too - no annoying limitations, nag screens or anything else. Epic now only requires that you pay a 5% royalty after the first $3,000 of revenue per product per quarter. And even then, you "pay no royalty for film projects, contracting and consulting projects such as architecture, simulation and visualization."
- Bug Fixes
A professional set of development tools which can produce just about any game or simulation you can imagine (and plenty you can't)