One way I saw it done was that if you got 5 over a target's AC(rolling a total of 20 with someone with a 15 AC) you could pick the general location of your attack if you wanted: Upper body or lower body. Then you rolled a D6 to choose where it landed, for upper body it would be like 1-2 left arm, 3 chest, 4 head, 5-6 right arm. If you did enough damage you'd get a bonus with your attack. If you got 10 over a target's AC, you didn't need to roll a die on where you hit. Rolling a natural 20/Critting allowed you to do the same thing. I remember cutting a zombies legs out from underneath it with this system. I could have described it a little wrong, it wasn't my system, but I played with it.
