Sheesh, where to begin. I think aside from the 3.0 Core books, 99% everything I've bought has been because of a recommendation or discussion here.
Magical Medieval Society I probably wouldn't have bought otherwise, and it and its siblings are great. Grim Tales is a favorite. I got Black Company for a PbP game here. The Tact-tiles I got for the first ENWorld DC Gameday. Dicebags and book covers from The Gamers Bag because of their on-line personas here. I'd never have found RoninArts PDFs without ENWorld.
The Serenity RPG is the only thing I can think of that I've bought without reading about it here first.