3rd party stuff I have used a bunch that come to mind
Adventures: Goodman Games DCC, Demon God's Fane from Malhavoc, a free one from a defunct company, Freeport Trilogy, Harvest of Darkness from Kenzer,
Spells: Complete Book of Eldritch Might, Arcane Strife, Spells & Magic, Quintessential Wizard, Spells and Spellcraft, Into the Blue,
Feats: Traps and Treachery, Arcana Unearthed, Witch's Handbook, Complete Guide to Wererats,
Classes: Cityworks, Arcana Unearthed, Iron Heroes, Elements of Magic Lyceian Academy, Witch's Handbook, Deeds Not Words, Everquest RPG PH, Complete Guide to Wererats, Character Customization,
Prestige classes: Complete Book of Eldritch Might, Librum Equitis Compiled, The paladin book by Chainmail Bikini Games, Crimson Contracts,
Races: Mythic Races, Complete Minions/Oathbound/Wildwood, Arcana Unearthed, Aasimar & Tiefling, Book of Templates, Tome of Horrors Revised,
Settings: Oathbound, Wildwood, Ptolus, Freeport (as seen in the Trilogy), Diamond Throne, Raveloft CS, Twin Crowns Campaign Setting,
Rule variant inspirations: Iron Heroes, Arms & Armor 3.5, Everquest RPG PH, Arcana Unearthed, Ravenloft CS,
Monsters: Complete Minions, Slayer's Guide to Demons, Encyclopedia of Demons and Devils I & II, Book of Fiends, Penumbra Fantasy Bestiary, Denizens of Darkness, Creatures of Freeport, Monster Geographica Forest, Monsters of Norrath, Roll Initiative Monsters, Dice Freaks Hell stuff, Lost Tribes of the Scarred Lands, Book of Templates,
Gods: Lore of the Gods, Diamond Throne, Ptolus, Complete Minions, Wildwood, Encyclopedia of Demons and Devils I & II, Book of Fiends, Creatures of Freeport, Dice Freaks Hell stuff, The Atlas games cult book, Dragonlords of Melnibone,
Description stuff: Bits of Darkness Caverns, Bits of Wilderness Wildwood,
Counters: Fiery Dragon counters, Dragonscale games counters,
Ritual Magic Systems: Relics and Rituals, Heroes of High Favor Elves, Twin Crowns Campaign Setting, Witch's Handbook, St. John's College of Abjuration,