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.
Amazing range of platform choice to build to (all with separate licencing so you aren't paying for something you don't build for). Not to mention there is a free (slightly limited) licence for pc, mac and linux.
This engine takes pure advantage of component model and allows developers to truly design something modular, which is my personal end-goal in program design.
Every developer should at least have a look at what this engine offers to see the advantages it offers over the others.