Unity is a multiplatform game development tool, designed from the start to ease creation. A fully integrated professional application, Unity just happens to contain the most powerful engine this side of a million dollars.
Unity by Unity Technologies is AMAZING. It works for Microsoft Windows, Mac OS X, Virtual and Augmented reality devices, variations of Linux, web browsers, Adobe Flash Player, iPhone, iPad, Android, Xbox 360, PlayStation 3, the Nintendo Wii, and the Nintendo Wii U.