Things are getting diverse for rules sources...
3.5e Core Rulebooks
Expanded Psionics Handbook - psychic warrior, prestige class
Monsters of Faerun - monsters
Monster Manual II - monsters
Book of Vile Darkness - templates and spells
Races of Stone - feats
Complete Warrior - feats
Complete Divine - feats
Book of Eldritch Might II - alternative sorcerer
Tome and Blood - prestige class, spells
Living Greyhawk Gazetteer - setting, deity choices
Dungeon Magazine #114 - adventure, "Torrents of Dread"
In many cases, it's just one feat (e.g. Practiced Spellcaster from CD and Heavy Armour Optimisation from RoS, but they're significant).
Cheers!