I have long enjoyed the hit location mechanics in games such as RuneQuest and Warhammer, as I feel they add a nice element of further excitement and deadliness (discouraging murderhoboism) to combat and other perilous situations. Additionally, e.g. missing limbs can provide nice conversation pieces in-game and otherwise enhance roleplaying.
Most of the burden of book-keeping falls on the players, for a change. Probably counts as a plus as well…
Being unsatisfied with the d20 stuff I found online on the matter at hand, I came up with my own system, which you can find as the linked PDF below. Most of the (few) game-dependant mechanics are for DCC RPG, but I have mentioned some alternatives for various editions of D&D as well. In any case, converting this system between most d20 games should present little difficulty.
It is naturally up to the Judge/DM/GM how completely they want to adopt the system into their games. For every creature in the world? Just for the PCs, major NPCs and epic opponents? Other:_______________?
D20/DCC rules – Hit Locations