This game engine was used to develop some very popular games of late (http://unity3d.com/gallery/made-with-unity/game-list) including Blood and Glory, Jelly Defence, ShadowGun, Battleheart and Muffin Knight.
I think the basic Windows dev tools are always free but till 8th April so are the IOS and Android dev kits.
Speak of the devil, I was looking at buying the android one a couple of days ago, and I've been fiddling around in unity for the past hour or so. Bargain of the year so far for me!