Here is the critical miss table we use in my games:
Melee - roll 1d10
1-2 : You have overextended yourself. Lose 1 attack at your
highest bonus next round.
3-4 : You have left an opening in your defense. Opponent
gets a free AoO against you instantly. This does not
bestow your opponent with an extra AoO.
5-6 : Fickle finger of fate. Through some accident, you have
managed to wound yourself. Take 1d10 points of
damage.
7-8 : Fumbled! Your weapon slips from your grasp and
tumbles to your feet. Be careful next time.
9-10 : GM Choice. Don't make me angry. You won't like me
when I'm angry.
Ranged - roll 1d10
1-2 : You have overextended yourself. Lose 1 attack at your
highest bonus next round.
3-4 : Fumbled! Your weapon slips from your grasp and
tumbles to your feet. Be careful next time.
5-6 : Fickle finger of fate. Through some accident, you have
managed to wound yourself. Take 1d10 points of
damage.
7-8 : Dropped your ammo! Crap! Requires a full-round action
to gather it back up. In the case of spells, you just
dropped your components.
9-10 : GM Choice. I hope your friends like you. Cause you
just shot one.
Critical Misses utilizing Unarmed Attacks resut in 1d6 damage in the case of a result of 7-8.
Critical Misses involving rays, energy missiles, and a few other spells result in 1d6 damage in case of 3-4.
Its fairly simple, and not too deadly. We don't use any special rules for critical hits, but I have two tables for such on my computer (most likely scavenged from these boards ... if you are the author of one of these I'm sorry, I didn't keep track of your name!

)