In part of my ongoing after to rebalance 3.5 d&d, I have introduced the following variant into my own system:

The 2d10 Variant:

-Whenever you would normally roll a d20 for task-resolution purposes, instead roll 2d10. -If you get a result of 20, roll a d20 to confirm your critical normally.
-If you get a result of 2, roll a d20 to confirm your critical failure. If your roll fails to make the DC required for success, you critically fail.
-If you roll a 1 on a critical failure confirmation, you catastrophically fail and your DM is encouraged to invent an extremely bad result of your attempted action. For instance, if you catastrophically fail an attack roll, you could instead roll to attack your ally, who would be denied his Dex bonus to AC against your attack, and the resulting confusion would cause you both to provoke an Attack-of-Opportunity.
-Any reference to consequences to “rolling a 1” on a d20 instead refer to “confirming a critical failure”.

If you use this variant, you will also want to change the critical values on weapons as follows:

20/x2 -> 20/x3.5 (calculate as if x4, but deal only half damage on the final ‘hit’)
19-20/x2 -> 19-20/x3
18-20/x2-> 18-20/x2
20/x3 -> 20/x5
20/x4 -> 20/x6

-Criticals now are significantly nastier, but also have drastically less likelihood of occurring.
-Factors such as keen or Improved Critical still work as-written.
-Critical-dependant abilities such as Flamming Burst deal more damage as indicated by the increased critical multiplier.
-Vorpal now is only +4 market price modifier.

I dislike the amount of luck inherent in the d20, and feel it also spawns problems with how often people can really screw-up or fantastically succeed. Yes, these things do happen, but it's not 5% of the time, it's like 0.5% of the time. My system more accurately reflects this.

Also, this indirectly nerfs casters and buffs martial classes because its 1) easier to roll average to above-average on a save and thus not tank a save-or-screwed. And 2) You are more likely to roll average and hit on your attacks, making your combat options much more consistantly effective.

Please do note that in part of my work I have done extensive spell patching and moderately extensive class work as well.