WotC:
Masters of the Wild - Some high-end fighter feats, finally! (and not in the fighter classbook...)
Psionics Handbook - Well balanced, packed with goodies, and pretty flavorful for a rule book.
The DMG, Manual of the Planes, Oriental Adventures, and the Epic Level handbook - All of these to some extent showcase the "behind the curtain" mentality that in my mind is one of the best innovations of 3E.
Non-WotC:
KoK Player's Guide - The variant classes are fun, even if some could use tweaking/clarification. The sections on religious texts and customs and additional positive energy channeling uses really got the ides going for my homebrew campaign.
If Thoughts Could Kill - More and better psionic stuff, and the price is right.