Although I "steal" from just about any book I happen to have in some degree or another, I think the one I do so most often is Ravenloft. Whether in 2nd or 3rd edition, my undead and werecreatures having generally come with Ravenloft rules attached onto them.
I've also considered using the VP/WP system of some d20 games, the Heroic paths of Midnight in other settings to help diminish magic item dependence, and a class-based Defense rating, among other things. I have yet to institute any of those, though; but magic items, prestige classes, monsters, and other simple ports from a variety of sources have all made their way into my games.