I like DCC's Mighty Deeds mechanic. For those not familiar with it, the Fighter gets a die (d4 at level 1) instead of a static modifier to add to his attack rolls. Any time the fighter attacks, he can announce his intention to perform a mighty deed and if he rolls 4+ on the attack die, he succeeds.
It's simple and free-form, yet restricted enough that the fighter isn't poking out eyes every round.
Inspired by DCC's rule, I've been using a fudge die for similar effect--anyone can announce a 'stunt' (which includes called shots) as part of their attack and roll a fudge die beside their normal to-hit die--if they hit and get a plus, their stunt is successful. They will still do full damage, even if the stunt falls short of delivering the full effect.