I like to use the system found in Mongoose Publishing's the Quintessential fighter. It revolves around trading in critical hits for special flashy called shots (the head, the arm, etc), which they list in the book.
Each of these called shots has a description that lists the effects and the prerequisites of the 'caller'.
Basically you have to score a threat against the opponent, and you deal damage worth of one attack. You don't roll to confirm the crit, nor do you double your damage if it's succesful, but instead you 'call the shot' (ie. to the arm), then roll for another attack, and if that's succesful, the opponent suffers the penalties given in each 'shot' description (in the case of the arm a -4 to attack, the hand gets disarmed automatically and the attack only deals 1d4 damage). In order to make this called shot, the 'caller' needs a base attack bonus of at least 2.
Of course it doesn't have to be just the attacker that calls the shots (

), the DM could use it too for cool-to-watch effects.