The IW Engine is a game engine developed by Infinity Ward, based on id Tech 3, which was first used on the highly successful game Call of Duty 2. Since then, it has been continually upgraded and used on the games Call of Duty 4, Call of Duty: World at War, Call of Duty: Modern Warfare 2, and Call of Duty: Black Ops.
Shadowmapping, phong, dynamic lights and shadows, impressive animations, and physiscs all rolled into an engine that's about as technically demanding as Left 4 Dead. It's a nice little engine. I think all of the hate for this engine just is sprouted from the hate for Call of Duty.