Unity 3D has won the Wall Street Journal 2010 Technology Innovation Award in the software category, and Unity Technologies was named one of Gamasutra's "Top 5 Game Companies of 2009." Why? Because it's good.
What's Unity, and why's it free? Moreover, what do I need it for?
Unity 3D is a Game development environment and Game Engine, technical term Middleware. Middleware is the software that game developers use to build their games on. It might surprise you to know that they rarely code a game from the ground up. They use Middleware, which is a combination of 3D rendering, Physics, sound, animation, artificial intelligence engines, and more. The game developers use this software and then pay the Middleware developers to allow them to ship their game with it. It runs in the background making all the bits and pieces work.
The reason you need Middleware is obvious, and the reason you need Unity 3D is that it provides arguably the best and most well-supported free one out there. There are some differences between the free and pay-for versions which you can check out at http://unity3d.com/unity/licenses.
What this means really is that you're going to learn how to develop levels and assets for a true professional game environment. You will get an insight into how it all works if you were to work for a software house. Of course, these skills will naturally set you in good stead for all the less-demanding markets such as game-level modding. This is where you use simple tools bundled with commercial games to alter parts of the game or add to it.
Let's download it and see what a satisfying mess we can make in the pro gaming arena.