Dungeon Magazine
I also get a lot of mileage out of City books and modules with good cities/towns/viallges in them, such as Morricks Mansion, City State of the Invincible Overlord, Durbanford, Lost City of Barakus (Endhome), Bard's Gate, Grey Citadel (Don Eamon), Mithril (City of the Golem), Hollowfaust (City of Necromancers), and many others.
Wurst of Grimtooths Traps and the two Traps and Treachery books have been very useful. I also get a lot of use out of Alchemy and Herbalism.
I also have gotten a lot of mielage out of Goodman Games Complete Wererat, Lich, and Vampire books.
I am definitely getting serious bang for my buck with my Castles and Crusades books and modules, plus the Erde setting (which I got from the Trolls for $5.00) plus all their old modules they had (have?) on sale for $1.00 each.
I'm pretty sure I will be adding the stuff I won in my ENNIEs auctions to the list, such as the Goodman DCC's (hadn't bought any of them yet), the Iron Heroes auction, and the Harn auction. Plus the seat with Tabletop Adventures SWAG looks promising too.