....I assume this means Oriental Adventures is out. :\
In that case:
1) Tome of Battle: Book of Nine Swords (yay wuxia/ninja/anime style combat maneuvers)
2) Manual of the Planes
3) Psionics Handbook
4) Sword & Fist
5) Tome & Blood
Despite the errors and bits of overpowered material, I generally liked S&F and T&B a lot, and found them useful and informative 3.0 resources for warriors, mages, and related stuff. I liked the organizations, items, and prestige classes too. I consider the PsiHB to be less broken and wierd than the XPH of 3.5, needing few houserules for me to use it effectively (and none of that crystalpunk crud that crept into 3.5; before the XPH it was just a bit of crystal use in psionic items, and the psicrystal). MotP has lots of useful stuff, even though I would've preferred a full-blown Planescape 3rd Edition boxed set. The Tome of Battle is possibly the only 3.5 product I'll actually purchase, once I have a steady income again. The Draconomicon and Lords of Madness are possibilities too, but wholly dependant on whether or not I value their roleplaying flavor enough to buy them for use in my 3.0 games.
Fiend Folio, Monster Manual II, and the Living Greyhawk Gazeteer would be on my list, if I were sure they fit the OP's query. And Oriental Adventures would be at the top if it weren't for the fact that it doesn't fit the OP's query, being mostly Rokugan-specific in its material.
I might consider DMG II and PHB II if they weren't very-3.5 specific and half-worthless to me.