Version 4.0 of the Unity platform is available for purchase and download today. This is important, because eventually every game not made with the Unreal Engine will be made with Unity. I am only exaggerating a little bit.
Unity is a cross-platform development engine that allows for developers to develop a game on one platform and then deploy it anywhere -- smartphone, PC, Facebook, tablets, particularly technologically advanced house plants -- anywhere. It's a key player in the movement to create platform-free games. Games like Panglore's Knightly Adventure. Not just ports, but the same game running everywhere. It's the future.
Version 4.0 of Unity adds powerful new features to the platform, including DirectX 11 support and a preview of a tool allowing games to be published to Linux desktop. Mmm, Linux.
Want to see just how powerful Unity is? Check out The Butterfly Effect, a short film by Passion Pictures rendered in real time using the engine. It's mind-blowing.
Want to poke about with Unity? You can download the free version here.