I frequently use the following companies books in my campaigns.
1. Atlas Games: Occult Lore, Dynasties and Demogagues and Crime and Punishment along with their first hardocver (Touched by the Gods?), have seen a lot of use either directly or indirectly in my campaign.
2. Green Ronin: I couldn't count the # of their books that I've used. Noble's handbook for both player's and NPC's, testing out the Psychic's handbook to see how it compared to a psion, Hammer & Helm or Plot & Poison, which both still get a lot of use at my table.
3. Necromancer Games: I'm normally not too crazy about their adventurers, but I've used the Tome of Horrors 1 & 2 many, many, many times. I liked some of the material in City State of the Invincible Overlord too, especially the books section and some of the races.
4. Sword & Sorcery Studios: I love Warcraft. I used to really love the SL. Various mixed opinions about some of their other recent books, but I still use material from the various Player's Guide to XX that I'm putting them here.
5. Mongoose: I know people are going to think I'm crazy. I'm not saying all of their products are great or even good. Lord above knows that I've bashed them on many occassions. However, when a company puts out several licenses, I've got to give them props. Hold up Arcana Unearthed and Conan to a non role player and ask him which one he's heard of. Heck, do it with AU and Babylon 5, or Judge Grim.
6. Malhavok: I'm not too crazy about their smaller books as I tend to like larger books, especially in hardcover, but I've got much enjoyment in reading alone to not put them in a top ten list. I still reference the Eldritch Might books and found much in AU that I liked (Unfettered and Warmain are excellent and logical additions to a d20 game.)
7. AEG: Some of their one word title books were great, some terrible, but once again, this is more a thanks for the L5R and SA stuff that they dual statted. I seriously feel that if they had more hardcover books and tried less to keep to their old splat formula, that they'd still be doing d20 books.
I know I'm missing some companies and I apologize but my brain is swimming with the sheer weight of the many companies whose products I use as opposed to enjoy reading but never use. I'll see if I can get back to this thread latter. I know for instance, that Wizards of the Coast should be up here. For better or worse they are official and have still done a lot of good. Unearthed Arcana gets a tremendous amount of use in my game for example even as Complete Divine is only looked at when a player of a cleric or paladin feels he needs a little extra something.