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.
Good news everyone, Modern Warfare 3 will have dedicated server support for PC players!
We're a little late to the party on this one, but the biggest current FPS franchise in the world is bringing back mod support!