For my homebrew I need...
Monte Cook's Arcana Evolved - I love the races in this book, and the classes while not used by my players has made and interesting villain or two.
Chaositech - I changed this up somewhat for my homebrew, but the devices serve a extremely useful purpose in my game. It also makes them more accessible to PCs that normally don't like the corrupting nature of these items. The Body mods I use for villains to give them a nice buff and to make them seem more crazy (who would willing do that to themselves, crazy cultist). I use the Dungeon: Gamma World article for mutants.
Dragon Star - Need the tech and modern feats, although it occurred to me I might be able to just use SW Saga for this.
Midnight - The magical backgrounds (i forget what they are called) are really useful for creating "Chosen of" templates or uniquely powerful NPCs, without going to crazy.