A house feat I developed especially for this thread. I purposely limited it to mid-to-high level characters to keep limb-chopping from being a common occurrance before the Regenerate spell is available. There is also an alternate mechanic for severing limbs - the "Normal" option.
Sunder Limb [General]
You may strike at an opponent's limb to cripple the foe without necessarily killing him.
Prerequisites: BAB +9, Str 13+, Power Attack, Improved Sunder.
Benefit: As a standard action, you may attempt a single melee attack on an opponent's arm, leg, or similar appendage (this cannot be a limb required for survival, such as a neck or head). This attack is against the foe's full AC, and a -4 penalty applies to your attack. If you hit, you inflict normal damage or damage equal to 1/4 of the foe's maximum HP, whichever is less. If you inflict 1/4 of the foe's maximum HP, that limb is considered to be severed.
A severed limb (obviously) cannot function. A creature with a severed arm cannot use that limb's natural attack, a shield, an off-hand weapon, or a two-handed weapon unless he still has two or more arms still available. A creature with a severed leg is limited to 1/4 normal speed (or 1/2 normal if he still has three or more legs, or 3/4 normal if he still has five or more legs, or full movement if he still has seven or more legs) and must make a Balance check each round with a DC equal to movement in feet to avoid falling prone at the end of that movement. A creature with no legs can only move as a full-round action, and can only move 5 ft per round.
This feat has no additional benefit against monsters who have body parts normally vulnerable to Sunder, such as hydras. The DM must use discretion to determine the effects of limb loss other than arms and legs.
Normal: Without this feat, the character can sever an opponent's limb only as an alternate outcome to death on a coup de grace attack.
Special: A fighter may choose this feat as one of his fighter bonus feats.