I just created crit and fumble tables for my campaign, thought I'd share.
The idea is that you confirm your crits and fumbles as usual, and then you roll on the according table. A confirmed fumble is a roll of one, followed by a confirmation roll (if the confirmation roll would normally hit the target, the attack simply miss).
Crits damage still gets multiplied, but you roll on the table before doing so, as the damage might be modified. It's possible for a player, when attacking, to roll everything at the same time i.e. d20, damage dice, and d12.
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Crits & Fumbles...
Crit:
1d12 (if result doesn't apply, add d12 roll to damage before multiplying)
1- Crit is instead a normal attack (the blow LOOKS devastating, but it isn't)
2- Regular crit
3- Target is dazed for one round
4- Target is confused for one round
5- Target is flat footed for one round
6- Target is prone
7- Target is dazed for one round
8- Target is stunned for one round
9- Target is blown away, prone, blown away damage is lethal, sickened for ten minutes
10- Target looses an arm (exhausted until fully healed) *
11- Target looses a leg (staggered, balance check DC 15 to stay up each round, until leg regenerated) *
12- Facial hit, blinded (until regenerated) *
* These results when suffered by player characters (or NPC's the GM deems so) aren't as bad. The leg/arm/eyes aren't actually gone, but crippled, and the character recovers fully when all his hit points have been cured.
Fumble:
1d12
1- Roll twice
2- Ankle twist, staggered for 10 minutes or until fully healed
3- Critical hit friend
4- Critical hit self
5- Normal hit self
6- Stunned for one round
7- Prone
8- Flat footed for one round
9- Confused for one round
10- Regular miss
11- Attack actually hits
12- Attack actually hits and is a critical
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I just HAD to find a use for that fucking d12... :p