In another thread, I mentioned Critical Hit and Critical Fumble decks. Does anyone else use these or a similar resource? Basically, instead of just multiplying damage on a critical hit or a natural 1 resulting in automatic failure, you end up with dozens of potential results for each crit or botch roll. They are dependent on weapon type: for example, the Fumble cards split up the rersults based on whether you attacked with a melee or ranged weapon, a natural attack or a magical effect. The Crit cards are based on damage type. Sometimes, a crit card doesn't multiply damage at all, but instead presents a variant effect: the target may be sickened or stunned for a round, or maybe you get an extra attack. In my previous gaming group, I instituted a house rule that said a character couldn't suffer the effects of a fumble more than once a turn. This SHOULDN'T matter, statistically: there's only a 5% chance you will roll a natural 1 on an attack, and then you have to fail the confirmation roll, too. But one of my former players has notoriously awful luck with dice and actually confirmed two botches in one round, one of which was confirmed with a second natural 1 (which has something like a 1 in 400 chance of occurring). The decks are designed to make critical failure more interesting, not more deadly, so I had to make a ruling to protect him from himself (in that particular round of combat, he shot himself in the foot then spilled the rest of his arrows out of the quiver. Everyone was yelling at him to stop attacking).
The decks also include optional rules like new spells that make characters more likely to fumble their rolls, as well as clarification about how something like bleed damage works (which plays a prominent role in Pathfinder but doesn't come up a lot in v3.5).