I use my own set of Critical Hit and Fumble charts. They are under construction and ideas for the chart are always welcome. Keep in mind anything mentioned also applies to the PCs when they crit either way.
I am introducing a disfigurement set of rules. For each point of damage into negatives a character goes, he has an 8% chance to be disfigured depending on the attack that damaged him. For example: a sword attack brings a character to -3, he has a 24% chance on the slashing chart to be disfigured. If a character is in the negatives and is attacked and loses more hp, the new negative number will determine the chance of disfigurement. I believe this option adds a bit of flavor.
I am implementing a reward system called Hero Points. These hero points are a currency for players to use to aid their characters in a temporary sense or burn them and improve the character permanently.