I'd started this topic on another, now apparently defunct, D&D website. Just as the title suggests, I want to know who's using homebrew or house rules, what they are, and how or why you came up with them. Here are some of mine...

COMELINESS: I've been using this rule since 1st edition. It was published in Unearthed Arcana, and is a 7th stat which is rolled to represent your character's physical beauty. It's rolled using 3d6, same as the other stats, then adjusted for race and Charisma. Half-orcs get a -3 penalty; gnomes and dwarves -1; Humans and halflings have no adjustment; half-elves and wood elves get a +1 bonus; gold elves, silver elves, and drow get a +2 bonus.
Then CHA: 2 or less gives a -8 penalty; CHA: 3 gives a -3 penalty; CHA: 4-5 gives a -2 penalty; CHA: 6-8 gives a -1 penalty; CHA:9-12 has no adjustment; CHA:13-15 gives a bonus of +1; CHA: 16-17 gives a +2 bonus; CHA: 18 gives a +3 bonus; and CHA:19+ gives a +5 bonus.
Anyone who sees a character with a COM score of 16 or more who's WIS score isn't a specific % of the COM score (the higher the COM, the lower the %) is automatically affected as if by the 2nd level illusionist spell Fascinate (sort of a weaker version of Charm Person) unless they make a successful save vs magic, adjusted for WIS. My old DM changed the saving throw to a save vs rods; staves; and wands as a phallic joke. We all thought it was so funny that we adopted that as the new saving throw (we were only teenagers at the time).
I continued using this rule right through 2nd edition, and it can be easily adapted for use in 3.X, and soon 4th edition (changing save vs magic to save vs willpower for example).

VOCAL COMPONENT: I have a rule that all players who are playing spellcasters (clerics, wizards, bards, etc) must speak the vocal component for any spell their character is casting. Thus, saying "I cast fireball at the oncoming orcs" and then rolling dice doesn't work. You have to say "I pull out a ball of bat guano, sprinkle some sulphur over it, and say 'LLAB ERIF!', and cast my fireball at the oncoming orcs". THEN you can start rolling dice for damage.

THE PAUL RULE: A member of my old gaming group, Paul, liked to play very intellectual type characters (wizards, rogues, bards, etc). The problem was he kept playing them in a hack & slash style. Rather than trying to think or role play his way out of situations, he'd always resort to violence. Also, whenever I'd try to add some real ROLE playing to the adventures, he'd whine and moan about talking to NPCs when they should be killing them, or even getting up from the table and going to watch TV while waiting for the "Real" part of the adventure to happen. I only put up with this crap because we were playing at his house, and he was the only one of us who had a place that could accomodate all of us. Still, he needed an attitude adjuster so I came up with this little rule. Every time he would start complaining about there being too much talk and not enough blood, or whenever he got up from the table saying "Call me when there's something to do", I'd deduct anywhere from 200 to 400 XP from his tally for the adventure. Eventually he learned to take part in the WHOLE adventure.

COMBAT XP: This rule may be obsolete with the creation of the Challenge Rating system, but I found that giving out the amount of XP listed with the monsters in The Monstrous Manual made players more hungry for hack & slash than for any real role playing opportunities. Things like my Paul Rule didn't have much effect at first because once they met a troll or giant they'd gain back any XP lost. Therefore I eliminated all monster XP (or as some of my players call them, "Kill Points") and only started handing out the class specific XP rewards (spell casting for wizards and priests, 10 XP/HD of monster killed for warriors, etc). Thus a fighter would only get 60 XP for killing a troll, not 5000 XP like the book says.

There are some of mine. Feel free to list some of yours. I look forward to reading them.