I picked up a nifty pair of hit location dice, with this idea of making combat more fun. Here is my story.
Why Hit Location?
I have a long history of tinkering with combat rules, mostly in response to my disdain for hit points. Back in the '90s, I created critical hit tables for my AD&D 2nd Ed. game, very much in the bloody style of Arms Law (e.g., "The arrow lodges in your eye socket, cutting off your ability to see or live."). They were fun to create, but they proved cumbersome in play and, honestly, not that entertaining beyond the third or fourth sight-robbing arrow.
When I switched over to the Rules Cyclopedia, I reworked critical hits into critical damage, which I have to say worked pretty well, even though it lacked the violent descriptions so thoughtfully presented in my previous system.
Fast forward to Chimera, which does not use hit points. Still, there's a place in my heart for crits. But maybe not critical hits. Hit locations.
Hit locations provide a few benefits over traditional critical hits. First, you avoid some of the pitfalls common to most critical hit systems (see OD&DITIES #11 for said pitfalls). Second, it doesn't rely on hit points (but it can use them if your system wants). Third, it gives flexible results, which you can port to pretty much any game you play and scale however you like.
How it Works
The hit location die is a d6 with pictures of body parts on each face. Like this:
You'll want to devise a quick table of results for your system. Conventional wisdom demands that the intensity of these results is proportional to the importance of the body part hit. For example, "Head" = crucial (implying bad things when it's hit), while "Leg" = largely disposable (meaning it will hurt less).
Results should be general—the less specific, the better. This is because we're interested in fast outcomes, there are only six results, and you may want to use this for more than one system without reinventing the wheel each time you translate it. If there's one rule of thumb, it's that results should not require more die rolls.
The lowest common denominator is to alter the damage dice according to the hit location struck—this can apply to any system. If you want to get fancy, you can add some special effects specific to your game. Or, maybe add truly dire things when you roll the maximum result on the damage die. Here's an example you can easily apply across various systems:
Hit Location Die
1 (left arm)
Items dropped if attack roll is divisible by 5
Broken (useless until healed)
2 (right arm)
Items dropped if attack roll is divisible by 3
Sprain (–2 penalty until healed)
3 (left leg)
Knocked down if attack roll is divisible by 5
Broken (Movement Rate 1" until healed)
4 (right leg)
Knocked down if attack roll is divisible by 3
Sprain (–1 to initiative until healed)
Knocked back 1" if attack roll is divisible by 5
Internal damage (–1 penalty until healed)
Stunned if attack roll is divisible by 3 
Knocked unconscious 
1. Duration equals 1 round for each point of the attack roll in excess of required "to-hit" number.
2. Duration equals 1 turn for each point of the attack roll in excess of required "to-hit" number.
Roll the hit location die whenever you attack. If the attack succeeds, roll damage normally and apply Damage Adjustment and Special Effects based on the location hit. Max Damage results occur only when the damage roll is the highest value for that die (e.g., a natural "6" on a d6 or a natural "8" on a d8).
This system is deliberately fast and loose, designed for maximum portability. I used this with the Chimera RPG to good effect, but there are caveats I think might be applied regardless of which system you use:
The hit location approach can bog down combat with many participants. To keep things moving, use it only when fighting "level bosses," during single combat, or in otherwise important fights. In Chimera, I reserve hit locations for Personage-class opponents. Lesser foes (Fodder in Chimera terms) tend to be dispatched quickly and easily anyway, so using hit locations is unnecessary work.
Right and Left
The hit location die differentiates between right and left arms and legs, which is largely useless in game terms (the possible exception is your target's arm, which might be wielding a weapon or hefting a shield). On the other hand (no pun intended), this arrangement lets you assign two different results for arm and leg hits. I didn't get especially creative on the table above, but I did inject some variation in the Max Damage column.
You'll note that every location has a 1/6 chance of being hit. Normally, my inclination would be to give big odds to the torso while making head shots rare. But after seeing the system play out, the even weight across the d6 isn't really an issue.
In melee combat, limb shots are martial chicanery—if you really want to bring down your foe, aim for the head first, torso second. But arms and legs get in the way as combatants lunge, dodge, feint, parry, etc. So a 4/6 chance of hitting a limb doesn't seem so unreasonable. Similarly, in missile combat, you'll shoot at a vital area, but distance makes accuracy difficult. Again, a 4/6 chance of hitting an arm or leg—instead of the head or chest you were aiming for—isn't unrealistic.
Given those rationalizations, you probably don't need to revise the hit location die's weight.
This approach dovetails nicely with location-based armour (e.g., head = helmet; torso = breastplate; arms = bracers; legs = greaves), provided your system uses so-called "ablative" armour, which absorbs damage (Chimera) instead of determines armour class (D&D). If so, then you adjust the damage by the location's protection, which lets you reflect stuff like wearing mismatched armour or the perils of forgetting to don your helm.
Not Everything has Limbs
Though, surprisingly, the hit location die can handle snakes, plants, fish, wyverns, elementals, purple worms, and other limbless baddies. Consider:
Head: pretty much everything has a head, except for slimes and oozes, which have no real body parts anyway—problem solved.
Torso: whatever contains the creature's internal organs and is not the head; for plants, this is the stem or trunk.
Arms: could be upper-body appendages, like wings, tentacles, pincers, fins, flowers and vines, horns, antennae; treat as Torso if no appendages exist.
Legs: lower-body appendages, like tail, flippers, tentacles, stinger, roots and branches, ovipositor; treat as Torso if no appendages exist.
Do Not Complexify™
This is a simple system, ripe for expansion. Though I'd caution against it, mostly because 1) you want to keep play moving, and 2) anything you think is fun to inflict on a monster will annoy your PCs when it happens to them. Everyone likes to behead an ogre with a single blow; I've never seen a player laugh when someone tosses their character's noggin. 
I Didn't Need Two
I bought a pair of hit location dice, thinking that I'd somehow incorporate both (not that I have any idea what "left leg" + "head" would have meant...I think I was contemplating some counterstrike options at the time). Anyway, one die turns out to be enough, and I'm forced to admit that 80% of the reason I bought two was to satisfy my dice fetish. There, I said it.
The only thing I don't like about these dice are the sharp corners. All my other dice are round and smooth, which makes it easy to jiggle them in my hand before the toss. Most will agree that this is an important (if not essential) component of the dice-rolling ritual. Unfortunately, the corners of these dice hurt. My wife says it's because I have city hands, from counting money all my life. I say good dice don't draw blood.
Hopefully, this little system gives your combat some flavour. Even without the dice, you can map a d6 to body parts (though part of the fun is the full-colour body part pictures on the die). As always, comments welcome, especially if you get a chance to use it on your group's table.
1. Unless, possibly, he's drunk. After a few real-life snorts, a player's mage lost his melon to a vorpal blade. The player gleefully rolled up a new character, wittily named him "Snicker-snack," and spent the rest of the session talking—in character—like a parrot. The lesson here is that Dewar's 1) takes the edge off character loss, and 2) makes it possible to tolerate a friend speaking in parrot-voice for 3 hours.