Must have 3.5 supplements. . .

[Olaf the Stout]

Since you have said you are going to get the Spell Compendium, I would go for the Magic Item Compendium next. Pretty easy to add into your existing game as it includes updated treasure tables for you to use.

Olaf the Stout

 

[Jdvn1]

Dragon Compendium, Volume 1.

 

[Voadam]

I'm not looking for standalone OGL games (e.g., Iron Heroes which, BTW, I own). Also, I'm not looking for splats of new feats or classes. What I am looking for are books specifically designed to supplement the 3.5 core rule books during actual play (e.g., stuff like the Spell Compendium, The Mother of All Encounter Tables, adventures, etc).

So, educate me please, ENWorld

Battle Box by Fiery Dragon is highly regarded.

I use Cardboard dungeons and cavern sets from Steve Jackson Games for miniature mapping in tabletop use though the Dungeon Tiles from WotC look great for that as well.

Fiery Dragon's Counter Collections for printing out monster pics at miniature size. All the srd monsters plus plenty of NPC ones. Copy and print as many as you want.

 

[DreadArchon]

Yes, UA is *really* nice. I suggest PHBII next, though. Magic Item Compendium is nice, but you can do without it just fine, IMHO. (Then again, I wouldn't have taken Spell Compendium either--I prefer to have an incomplete list, spread over three or four books, over a nearly-as-incomplete "Compendium" that I had to buy separately and contains no other useable material than its highly limited specialty.)

 

[Greg K]

I personally loved Complete Mage for the MASTER SPECIALIST PrC as well.

I wanted to like that PrC, but I really preferred Unearthed Arcana's variant specialist wizard abilities.

 

[Mistwell]

PHB II
Unearthed Arcana
Magic Item Compendium.
DMG II

 

[Thomas Percy]

Must have? SRD or core.
The rest: Tome of Magic
DMG 2
Complete Warrior

 

[Mercule]

For what it sounds like you want, I'd recommend the Spell Compendium and Magic Item Compendium.

When SP first came out, I thought the idea of paying for a book filled with rules I already owned was absurd. A few months later, I picked it up on a whim. It's great. I referrence it at the table more than any book besides the core 3 -- and I'm not even that fond of spellcasters.

MIC is pretty much the same goodness, appied to magic items, plus some previously unprinted stuff and swell tables. I like both books enough that they would be my first two non-core picks of all suppliments, if I were limited to five books (or six, since I like psionics, too).