Friday night I downloaded my first mod for Call of Duty called Merciless Blood Mod. It's a great change in gameplay from the original game. No weapons have changed or map changes, but it does change how the player reacts from incoming fire, gunshot wounds, and the mod adds gore and death sequences. When shot, the player becomes disoriented and things seem to move in slow motion. The gun may be shot from someone's grip, leaving them helpless in a gunfight. Puddles of blood also stay where a player is killed. Pain sounds are now added, so you can hear when a player is shot.
To download the mod, you must login to their site in the bottom right, here is an account I setup for distribution.