Lesse . . . my house rules in a nutshell.
Well, I think up lots of house rules, but it's hard to implement some of them without player interest (for example, no one has wanted to play a Wild Spellcaster, alas, though they make for interesting NPCs).
However, I do use a modified wound point system. PCs get bonus WP equal to their Con (or 3/4 Con for small characters). WP heal at a rate of 1 per day (or 1 per die of magical healing). Normal hit points heal like old subdual damage used to heal, 1 per level per hour. I wanted to make it so clerics weren't necessary for a game to be playable, so I removed the need for magical healing.
Also, like many others, I don't use favored classes for multiclassing. I also don't use the class skill system; as long as a PC has a reason for learning something, he can spend skill points on it.
I created some of my own special mage-dueling rules, because one of the PCs wanted them. Made up a few new races, a few prestige classes. I suppose all the stuff I've written for Nat20 could count as house rules, too; whenever I give a PC a special power, I figure out its cost using Four-Color to Fantasy's superpower rules, and I've run a few games and drinking competitions using the stuff from Tournaments, Fairs, & Taverns.
No one uses Tumble, but if they did I'd use Piratecat's suggestion that the Tumble check sets your AC for any attacks of opportunity you incur while tumbling.
I've also adopted a system one of my fellow DMs uses. Whenever I, as GM, really don't care one way or another how something turns out, or when I want to see how bad a fumble is or how lucky someone is, I just roll d%. Low rolls are bad for the party, high are good. Say a PC wants to buy some healing potions in a small village, where there aren't likely to be any. I'd roll, and say on a 90+, he might find some, or on a 10 or less he might anger someone while he's trying to buy 'evil magic'.
Or, if a PC fumbles an attack roll, and I roll a 90 or higher, he might just get a -2 penalty to his next attack roll because he's off balance. From a 40 to an 89, he incurs an attack of opportunity from his foe. 11 to 39 he'd be flat-footed and incur an attack of opportunity, and 10 or less he'd probably lose his balance, fall down, and drop his weapon. Depends on the cirucmstances, though.