A few house rules.

[Dimirag]

I use the "Rule of 20" and "the Rule of 1" to "activate" different effects such as weapon effects like extra attack, trips, etc.
Besides that I use it as an "auto-hit".
Instead of double damage I prefer max damage because the nat 20 means the best attack possible but the damage represent the "attack quality", so max quality = max damage (or extra effect).
The Hackmaster Basic game use exploding dice (referred to "penetrating dice"), IIRC the backstab ability increase the "p-range" (the max number in the dice) by a given value.
I'm currently working on a Duelist Class which will use "the rule of one" on enemies attacks...

[dymondy2k]

I use the "Rule of 20" and "the Rule of 1" to "activate" different effects such as weapon effects like extra attack, trips, etc.
Besides that I use it as an "auto-hit".
Instead of double damage I prefer max damage because the nat 20 means the best attack possible but the damage represent the "attack quality", so max quality = max damage (or extra effect).
The Hackmaster Basic game use exploding dice (referred to "penetrating dice"), IIRC the backstab ability increase the "p-range" (the max number in the dice) by a given value.
I'm currently working on a Duelist Class which will use "the rule of one" on enemies attacks...

Dimi.. thats exactly how I do it as well.. nothing fancy on that 20.. just full damage. I did find a cool chart I've been considering that throws some other things into mix..

[MedievalMan]

I have seen that rule before about just doing max damage on a roll of 20. I think I am going to do it that way as well.

*Edit*
Funny how everyone has a rule on critical hits but nothing about fumbles.

[Dimirag]

The "max damage" rule is elegant, fast and simple compared to the:
1-Roll to hit
2-confirm the crit
3-roll double dice for damage or double the damage roll result
Which can turn on this:
1-rolling for attack... 20! yeah!
2-confirming critical attack.... HELL YEAH!!!
3- rolling damage... what seriously? a critical attack of... 4pts???

[MedievalMan]

Yeah exactly haha It always seems to come at the wrong time too.

Player: I need to kill this dragon! Its taken down the rest of the team "rolls" Ah yeah critical hit!
DM: Ok roll to confirm.
Players: "rolls" Yeah! I confirm!
DM: Ok roll damage and multiply it by 2.
Player: "rolls" Alright, this is it. "Dice says 1" .........
DM: Ok adding that up you deal 2 damage. Next.

[SmootRK]

I am really liking the idea of criticals meaning max damage. However, simple 20 = crit does not float with me (as already detailed above)... I can envision something simple like roll X over the number needed to hit means a crit (max damage). It could be simple, like 4 over (after modifiers for str, magic, etc) makes the crit. The number might need to be more or less, but it would reinforce Fighters (having the most combat modifiers STR, magic weapons, specialization if used, etc) as the ones that score the most crits (a logical thing).

If you need a 19 to hit, you ain't going to get a critical... but if you are fighting joe the retired adventurer who is unarmored and holding only a meat cleaver (AC 11) odds are you will beat him handily if you have a couple of levels.

And, perhaps the idea can be expanded upon, if you score Y over the needed number to hit, then you get full double damage... just an idea.

[Dimirag]

Similar to the d&d monk's stun attack (if you hit by 5 better than needed you can stun/kill your opponent).
I was thinking something similar but the number required on the d20 is based on the opponent's AC (the better his defense the harder to max out ones attack), I'll have to see how to unified this with my take on weapon/combat style specialization...

[MedievalMan]

I like that idea where you score a critical by rolling higher than the targets AC. Maybe along the lines of AC+5 or natural 20? Makes wearing plate mail a lot better. Interesting idea none-the-less.

[Dimirag]

Exploding Dice + Nat 20 + AC combo:
If the attack roll is a nat 20 roll again and add, keep adding as long as nat 20s appears, then then compare against AC.
You can have different effects based on the amount the attack beat the defense (per 5pts for ex.)

[Sir Bedivere]

I use a fumble table for natural 1s. If a player rolls a 1, he rolls another d20 and checks the chart.

Offhand I don't remember the chart exactly, but the effects go from something like 'You got lucky! No fumble.' for a natural 20 on to 'You disarm yourself; roll to see where it lands' to 'Your weapon breaks' to 'You hit yourself or a close ally (GM's choice / whichever is funnier)' to, for another natural 1, 'You hit yourself for maximum damage.'

I have also used critical hits on a natural 20 (max damage), but think I prefer the idea of rolling 5+ over the needed number for critical hits and 10+ for even better critical hits. I made the change in my house rules, but then my playing group stopped playing, so I never got to test that out.