I have fairly standard house rules such as a fumble chart for rolling 1s on attacks. And then I have other house rules that I use for certain kinds of campaigns to change the feel of the game such as critical hits for games that I want a more realistic feel or more radical changes for a homebrew campaign of mine where the characters jump from time period to time period in Earth's history. In this campaign the characters are able to switch classes with no restrictions and there are many different combat rules such as different pools for body points and stamina points. I also like to tinker with the rules a bit and try different things, all with the approval of the players of course.
"Plan?...There ain't no plan!" - Pigkiller