Critical Hits by the book. We do fumbles like we do Critical hits, if the player misses them on another roll, then we bust out the house Fumble Chart. The chart is % based and usually ends up with them dropping their weapon, provoking an AoO, or breaking their weapon (unless magical). We also added a balance check on fumbling two-handed weapons.
Here's something we as a group pondered as a house rule. If someone/thing gets hit with a critical and atleast half of their hit points are reduced, should we have them roll a balance check or a should they suffer from an additional AoO?