Third Party Materials
I mine a vast expanse of resources for stuff to fit into my game. I homebrew and I tinker a bit. Overall, I am fairly comfortable adding anything that I think will enliven my world. I have removed races, and adjusted races. My criteria though is that I feel it is powered for the game I want to run. That means it is relatively well balanced against the rest of the group, and it fits the atmosphere of my campaign. In the current campaign, let me run through the books that have material which has been used so far. I am just going off rapid memory, so I al almost certainly miss some things.
Core rulebooks
Psychics Handbook (Green Ronin)
Shaman's Handbook (Green Ronin)
Beyond Monks (Chainmail Bikini Games - In print via Goodman games)
Complete Warrior
Complete Divine
Swashbuckling Adventures (AEG)
Path of Magic (Fantasy Flight Games)
These are used almost every session since we have PC's with character classes from these books.
Other books that have seen, or are likely to see player use include:
Artificer's Handbook (Mystic Eye Games)
Spellbound (Living Imagination)
Book of Exalted Deeds
Occult Lore (Penumbra/Atlas Games)
Book of Hallowed Might (Malhavoc)
Book of Iron Might (Malhavoc)
Portals & Planes (Fantasy Flight Games)
Corsair (Adamant Entertainment)
There are many, many more books that I take material from. Most of it would be available to PCs in the correct circumstances.
Overall, I mine third party materials heavily. As it is, I have bought far more third party books then I have WotC books. I like to sprinkle new ingredients into the mix once I see something that feeds a new idea to me. I think many people would look at the variety of books I use and think I am completely out of hand. But I don't allow books into the game wholesale unless the entire book fits my vision nearly flawlessly. I am just careful to try to keep each PC with a niche that they excel at. There are a lot of books that can provide a little material here and there for PCs to benefit from. I encourage that and I tell my players that if they have a concept, or an idea, they should tell me so I can help them reach their aspirations.
Overall, I have been fortunate and I have not yet allowed something that broke down in play. So it has all worked out nicely.