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.
I love this engine. ^^
My only problem is that the free version has less rendering capabilities than the pro version. There really is no reason good enough to revoke that... I'm not gaining any unjust profit just because my engine renders shadows. ಠ_ಠ