in order of import ...
Ptolus - best $ ever spent (plus vinyl map)
Tome of Horrors I-III (I like ToH I so much that I bought the 3.5 PDF and had it printed and professionally bound ... also worth it ... and I'm by no means rich)
GameMastery Combat Pad initiative tracker - makes life so much easier
Spell Compendium
Eldritch Sorcery - great old school spells updated to 3.5
Complete Book of Eldritch Might - love the sorc and bard variants, standard IMC
Magic Item Compendium
Fiery Dragon Battlebox - very useful
AEG Toolbox
Traps and Treachery I
Minis - use Auggie's or CheapMinis on ebay to round out your monster/NPC archetypes by purchasing groups of 10 for pennies (orcs, ratmen, goblins, bugbears, gnolls, armored guards, archers, undead, etc) Warning: I personally have trouble resisting urge to blow my entire paycheck on rarer minis when browsing these ebay stores, so I sort by price and give myself a hard limit
Rappan Athuk Reloaded - modular dungeon levels that can be plugged into anything
Wilderlands of High Fantasy box - not necesarily for the (admittedly fantastic) campaign, but for the 1000's of well thought-out plot hooks, locations, and gaming ideas consolidated in a single place.
A Magical Medieval Society: Western Europe - these guys are intelligent
Dungeon Master's Guide II
Advanced Gamemaster's Guide
Penumbra/Atlas Dynasties & Demagogues - also an intelligent work
Dungeonscape
Gary Gygax's Insidae
From Stone to Steel - if you can find a copy in print, BUY IT
(ISBN 0-9728197-1-1)
A Magical Society: Beast Builder - even more useful than the following two tomes ...
Advanced Bestiary
Book of Templates
I'm inclined to agree about the inpropriety of FranktheDM's comment, though i'm sure the it came from a desire to be helpful. This is a gaming forum, not a mothering forum.