Ignoring the core, I'm a big fan of the following D20 fantasy products:
1) Savage Species
2) Book of the Righteous
3) Complete Warrior
4) Complete Arcane
5) Complete Divine
6) Complete Psionic
7) Complete Adventurer
8) Expanded Psionics Handbook
9) Hyperconscious
10) Dragon Compendium volume 1
11) Dragon issues that have yet to be compiled, like #341 (feats for Soulblades- some better than in CompPsi) or #319 (Oriental Adventures 3.5 update).
12) Midnight 2.0
13) Arcana Unearthed/Evolved
14) Spell Compendium
15) Book of All Spells
16) Oriental Adventures
17) Iron Kingdoms
18) Forgotten Realms setting
19) Eberron Setting
20) Nyambe
21) Northern Crown
For D20 modern (beyond the core):
1) Ultramodern Firearms D20
2) Spycraft 2.0
3) Mutants & Masterminds 2.0
4) Modern Magic
5) Second World
6) Urban Arcana
1) Savage Species
2) Book of the Righteous
3) Complete Warrior
4) Complete Arcane
5) Complete Divine
6) Complete Psionic
7) Complete Adventurer
8) Expanded Psionics Handbook
9) Hyperconscious
10) Dragon Compendium volume 1
11) Dragon issues that have yet to be compiled, like #341 (feats for Soulblades- some better than in CompPsi) or #319 (Oriental Adventures 3.5 update).
12) Midnight 2.0
13) Arcana Unearthed/Evolved
14) Spell Compendium
15) Book of All Spells
16) Oriental Adventures
17) Iron Kingdoms
18) Forgotten Realms setting
19) Eberron Setting
20) Nyambe
21) Northern Crown
For D20 modern (beyond the core):
1) Ultramodern Firearms D20
2) Spycraft 2.0
3) Mutants & Masterminds 2.0
4) Modern Magic
5) Second World
6) Urban Arcana