Some time ago, in response to a thread about giving 4E minions extra effects when they roll a critical hit, I suggested the following table:
Critical Hit Table (roll 1d8)
1 - Head strike: The target suffers blurred vision and treats all its enemies as having concealment until the end of its next turn.
2 - Chest strike: The target staggers backward and is pushed 1 square.
3 - Gut strike: The target is winded and grants combat advantage to all attackers until the end of its next turn.
4 - Interfering strike: The target is hit in the arm and takes a -2 penalty to attack rolls until the end of its next turn.
5 - Kneecap strike: The target loses its balance and falls prone.
6 - Foot strike: The target's movement is hampered and it is slowed until the end of its next turn.
7 - Arterial strike: The target starts bleeding profusely and takes 4 ongoing damage (save ends). This increases to 8 ongoing damage if the attacker is 11th or higher level, and 12 ongoing damage if the attacker is 21st or higher level.
8 - Vitals strike: The target is hit in a sensitive area and takes 5 extra damage. This increases to 10 extra damage if the attacker is 11th or higher level, and 15 extra damage if the attacker is 21st or higher level.
I didn't mention it at the time, but I actually had two other uses planned for this table.
First, I thought about using it as an alternate at-will attack (either replacing an existing at-will or "bought" with a feat), for those who want a bit more variety and unpredictablility in their combats. The PC makes a melee basic attack, and if he hits, he also gets an extra effect determined by the roll of 1d8.
Second, it could also be used as a called shot table. If the PC really wants to get a specific benefit, he picks that benefit and makes a melee basic attack. If he hits, he gets to roll 1d8. If the benefit he wants is within 1 number of the die roll (and the table "wraps round", so both 7 and 1 are within 1 number of 8), he gets the benefit. Otherwise, he just deals normal damage. The PC can increase the chances of getting a specific effect by taking a penalty to the attack roll. A -2 penalty lets him get the selected effect if it is within 2 numbers of the die roll, and a -4 penalty lets him get the selected effect if it is within 3 number of the die roll. The penalty cannot be increased further.
I had considered creating similar tables for encounter and daily attacks, but I wasn't really able to come up with enough distinct effects at the right level of power that would tie into the flavor of hitting a specific part.
Anyway, this is how I would do called shots in 4E if I really wanted to.