variant to variant
In an effort to create a better sense of "realism" to this rule, I have revised it slightly. The main differences are that it is a full-round action (Edit: changed back to standard action) and the penalties are slightly higher - but, they are divided by the number of "parts" the creature has and they stack with themselves. If this seems like a better way to go, then I will revise the innitial post to reflect it.
Called Shot
Players may attempt a Called Shot as a standard action against a living opponent with discernable anatomy (must be subject to critical hits). This provokes an attack of opportunity. If using a ranged attack, the player must be within 30 ft. of the target. Roll an attack using the penalty from the table below and calculate damage as normal. If the attack succeeds and any damage is dealt, the target must make a Fortitude save against a DC of 10 + ½ damage dealt or be effected. (Targeting the torso or "body" area is considered a normal attack and has no additional effect.)
Body Part: Penalty to Hit: Targeted Damage Effect
Head: -6: -2 all skills checks and will and reflex saves
Arm: -4: -2 to climb and swim checks, attack rolls, and Str checks
Hand: -6: -2 to all skills involving hand use and attack rolls
Leg: -4: -2 to all skills involving movement, reflex saves, and Dex checks
Foot*: -6: -2 to all skills involving movement and reflex saves, movement rate reduced as encumbered; **cannot run
Eye*: -8: -2 to all skills involving seeing, initiative checks, ranged attacks, and reflex saves; **blinded
Ear*: -8: -2 to listen and initiative checks; **deaf
If a creature has more than one of the body part listed, numerical effects are divided by the number of that body part a creature possesses, rounded down – however, these effects stack, up to the total number of body parts a creature has. For example, a Called Shot to the hand against a creature with 4 hands would cause no effect (2 divided by 4 = ½, rounded down = 0), but a second successful Called Shot would cause –1 to all skills involving hand use and attack rolls. 2 more successful Called Shots to the hand would cause this penalty to increase to –2. Further Called Shots to the hand would do nothing. Parts marked with an * cause extra effects (marked with **) once all of the parts of that type have been damaged; i.e. once a creature with two eyes has been effected in the eye twice, he is blind. At least one point of healing will heal all targeted damage from Called Shots, whether magical or from resting.