By:
Caliber
Natural 20 Critical Strike Table (Generic)
Roll Percentile
01-30: Double Attack (character gets a bonus attack on his enemy)
31-50: Double Damage (roll damage and multiply it by 2)
51-70: Double Damage and Stunned (as above plus loss of one attack by the defender)
71-80: Double Damage, Stunned, and Knocked Down (as above except loss of 2 attacks and initiative)
81-90: Triple Damage, Stunned, and Knocked Down (as above except damage x3)
91-99: Quintuple Damage, Stunned, and Knocked Down (as above except damage x4)
100: GM's Option (but should be brutally nasty)
Natural 1 Critical Fumble Tables (Generic)
Roll Percentile
01-30: Loss of 1 Attack
31-50: Loss of 2 Attacks
51-70: Loss of Main Weapon (if body weapon, then limb is stunned)
71-80: Hit Ally for Half Damage (if no ally, then loss of 2 attacks)
81-90: Hit Ally for Normal Damage (if no ally, then loss of 3 attacks)
91-99: Hit Ally for Double Damage (if no ally, then loss of all attacks for 2 rounds)
100: Lucked Out, Just Missed Real Bad
Now, of course it is the responsibility of the GM to make these interesting. You can just say "you did double damage", but the players are more likely to respond positively to "that last well placed hit smashed the beasty in his left hip, knocking him off balance and causing a brutal wound"...