Here's a little something I came across when going through some of my notes. This is something I had initially thrown together for Zombie Siege (currently in development hell) and will be appearing (in some form) in the Ballista. Nothing fancy here but simple and functional. I find it's pretty easy to remember or, worse case, even jot down on a sticky note for your DM screen.
On a critical hit, roll 2d6:
Roll - Result (Game Effect)
02 - Head shot / Right between the Eyes (Character Death)
03 - Weapon Arm / Primary Hand (Drops weapon or item. Triple Damage)
04 - Arm (Double Damage)
05 - Arm (Double Damage)
06 - Torso / Body Shot (Double Damage)
07 - Torso / Body Shot (Double Damage)
08 - Torso / Body Shot (Double Damage)
09 - Leg (Double Damage)
10 - Leg (Double Damage)
11 - Knee (Character Drops to Ground. Triple Damage)
12 - Right through the chest / Heart (Character Death)
Optionally, a roll of a two can be used for decapitations as well but the importance attached is death or near death. If you rather not kill a character, I suppose you could quadruple the damage, knock them unconscious, or just put them in a coma. Same thing for number 12, the intent was death but knocking a character out of the combat or ramping up the damage are options. Rolls of 3 or 11 could also be a simple severing of a limb if you are looking for something a bit grittier.
Why do this chart in this way? Well, there are 36 possible dice combinations with rolling 2d6 and of this, the 7 is the most common. On the other hand, to achieve a 2 or a 12, each only has 1 possible dice combination (2 ones or 2 sixes). The chart was built with these odds in mind.
Beware though, these could ramp up the grittiness of your campaign. Please enjoy responsibly!