Sovereign Stone: Codex Mysterium (Sovereign Press) IMO, a truly innovative approach to D20 magic, and the only book I've seen to actually detail how to design spells.
Freeport: City of Adventure (Green Ronin) An excellent campaign setting, period.
Shaman's Handbook (Green Ronin) A great take on pimitive spellcasters, and not afraid to slaughter a D20 sacred cow (the tendency of most people to not introduce new core classes).
Legions of Hell / Armies of the Abyss (Green Ronin) Excellent looks at demons and devils, without the disneyfication that the core books have. Erik Mona presents a system for the mechanics of demon worship that pulls no punches and avoids the vile for the sake of being vile trap that Cook fell into. These books show what a little reseearch can accomplish.
Hero Builder's Guide (Wizards of the Coast) Yes! I'm serious! Let's assume you've never played a pen and paper RPG before. Assume also that noone in your group has played either. Yet, you want something more than hack and slash. This book really gets the creative gears moving if you let it. A must have for any newbie, IMO. I make it required reading for anyone who wants to join my game, and also use it as a 'npc background machine' for those supporting characters who i dont want to bother giving a 2 or 3 page history from scratch.
Living Greyhawk Gazetteer (Wizards of the Coast/Role Playing Gamers Association) and Scarred Lands Gazetteer: Ghelspad (Sowrd & Sorcery Studios) These are what campaign setting books should aspire to IMO. Lots of information and plot hooks, little to no new rules crunch. A campaigns supplements can introduce a bunch of new rules later, that's fine, but in order to define the setting and make it appeal to the masses, I think it should be rules lite and flavor heavy. (and btw, in response to the 'sorcerors dont fit in greyhawk' comment, you've either not read the GH material and examined all the diversity and options in the setting, or you're afraid to bend the rules here and there in a setting to make things fit. To me, the fun of adapting to 3e has been fitting the new mechanics into the setting and my campaign. Anything, well almost anything, you want to fit into a setting, any setting, not just GH, can be fit in without ruining the setting if you do so logically.)