Last time in the dream world campaign we're playing in, my groups used the 3.5 core books, Player's Guide to Faerun, Occult Lore (dream creature templates), and the dream rules document we're playtesting.
In my 3.5 running of X4-5 (currenlty nearing the end of X4)
3.5 Core Books X4-Master of the Desert Nomads D&D Expert Set - used for reference for converting the Frost Salamander Miniatures Handbook--The War Hulk prestige class (using them as units in hte Masters army) and the Favored Soul class (one of the PCs) Defenders of the Faith (Holy Liberator PrC) Arms and Equipment Guide- the Axe Beak, being used as a mount (In my version several elite servants of the master ride Axe Beaks).
and a conversion document of monsters from X4-5 downloaded here at EN World.
I was a player in a Mutants & Masterminds Game (expect review soon!). the GM used Crooks, Freedom City, Mutants & Masterminds, and of course, the M&M GM Shield.
3.5 Core Rulebooks (all three- MM for some summoned monsters, DMG for item creation)
Tome & Blood
Manual of the Planes
Epic Level Handbook
The rest of the group (including the dm) also used:
Complete Warrior
Fiend Folio
Book of Vile Darkness
we sorta used the Forgotten Realms Campaign Setting (for some spells only, though the book wasn't actually present)
3.5 Core Rules (all three books)
Manual of the Planes
Book of Vile Darkness
Savage Species
Fiend Folio
Monster Manual II
Tome of Horrors
Expanded Psionics Handbook
Defenders of the Faith
Complete Warrior
Draconomicon