The trilogy of threads is complete!
Here are ten campaign setting books that I've been interested in for some time.
Warlords of the Accordlands: Master Codex (AEG)
Codex Arcanis/Player's Guide to Arcanis (Paradigm Concepts)
Codex of Erde (Troll Lord Games)
Midnight: Second Edition (FFG)
Morningstar Fantasy Campaign Setting (Goodman Games)
Nyambe: African Adventures (Atlas Games)
Sláine: The Roleplaying Game of Celtic Heroes (Mongoose)
Sovereign Stone Campaign Sourcebook [3.5 Edition] (White Silver Publishing)
The Hunt: Rise of Evil World Book (Mystic Eye Games)
Twin Crowns: Age of Exploration Fantasy (Living Imagination)
New Mechanics.
Master Codex is actually the player's book for the setting, the atlas is the campaign setting. The Codex has a lot of descriptions on the races and classes that flesh them out hugely though. I have the codex and not the atlas. It is mostly an alt PH giving most of the rules and such including tons of feats and spells and such. Plenty to raid for a homebrew.
Arcanis I recently got the 3.5 pdf but haven't read it yet. Neat thing about gods in the setting is the multiple aspects to each one, they can all have good and evil followers and have good and evil aspects to them.
Erde I got cheap ($4) but haven't read it yet, I think it has a new class in it.
Midnight I've got the 3.0 version and it has a lot of low magic options including a different magic system.
Morningstar I've got but haven't read it, I remember it sounding interesting but I never got around to reading it. Something about magic areas in the world.
Nyambe, ton of 3.0 crunch. Alt classes, feats that handle uncanny dodge and such, and a level based dodge bonus for the new classes and a corresponding feat for normal D&D classes to boost ACs in the hot armor inappropriate jungle setting. Lots of neat stuff like secret Ancient orc empire and serpent people stuff, Orisha spirits instead of gods, etc.
Slaine, an alt D&D system, their magic is very different from vancian. Based on the Celtic warrior comic. I'm interested in this as well but don't have it.
Soereign Stone, a different elemental non vancian magic system as well as an archer class. There is both a 3.0 (Sovereign Press) and 3.5 (White Silver Publishing) version of the game. The 3.0 verision has a neat separate bestiary book with Andy Hopp art and almost entirely beasties with no magic.
Hunt Rise of Evil, another I'm interested in picking up as well, a dark fantasy setting set in a pseudo dream realm and has some mechanics for dealing with the dreamers impact on the world. Plus I think a witch class. 3.0.
Twin Crowns is a 3.0 earth analogueish type setting with fantasy real world counterparts (Mostly european like Scottish, Russian, etc. but also Egyptian, Japanese, and others) Lots of mechanics to raid. Magic they have the best d20 ritual system out there IMO with fantastically flavorful big power rituals, later expanded in other books. Examples are things like making an instant fort, which their new world colonies use. Also pistols and cannons for ships. Most of their gods have dual aspects like the war gods' lawful tactician and chaotic berserker sides. Also a catfolk race.