Call of Duty, Gears of War and Halo all have free aim
What you normally do, is pressed the left trigger to shoulder (aim the gun) you will have a targeting reticle in the center of the screen, you then usually use the right stick to move your character to make your aim more precise (in my case, peoples skulls
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Free aim is much more exciting because you need skill and practice to be able to aim and avoid being hit, then aiming again when you lose sight of the enemy.