Torn Asunder is designed to be an add on for 3.5. I have it, I like it, it seems to fit your desire.
It works by using the normal crit system but adding effects if your die roll beats the necessary crit number (I think it is the confirmation hit number) by 5, 10, or 15. If your roll is sufficiently high for the extra crit effects you roll on a chart based on the target's body type (humanoid, quadruped, draconic, multilimbe, etc., usually a d8 or d12) look up the body section, and it will list three levels of effect for the amount you beat the roll by and the type of damage (bludgeon, slashing, piercing) and gives you a straightforward effect to apply.
More straightforward than rolemaster, a touch more gradation than WFRP.
There is also Paizo's critical hit deck, which doesn't give a body chart but has specific wound areas in the random results, with just the variability being in the damage types and the individual entries, so severe head blow is a card with differences for blunt trauma versus cutting versus spell I believe.
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). It's just chock full of ideas. It's very modular and adaptable, so you can just use the parts you want, and even tweak them quite easily to your own preferences. Even if you don't get it for a Critical Hits system, I'd highly recomend it. (The Martial Pool concept itself in the Codex is worth the price of the book.)
