The best "official D&D" deals are listed in the sale page:
Dragonlance Campaign Setting (3.5) for 4.99.
Dark Sun Campaign Setting (4e) for 5.99.
Planescape Campaign Setting (2e) for 4.99.
I'll analyze the 3rd party stuff in another post. Two of them looks good, and the other... not so much (let's see if you can guess which is which!). But I haven't read them.
Dark Sun, of course, is my favorite D&D setting. The Dark Sun Boxed Set (2e) is also on sale, for the same price. I have this one in print (POD - you can get a physical hardcover copy of the original for $23.99) and PDF and really like it. I'd pick the 2e version if I had to choose (although I might end up with both). The 4e version is also very good - I don't really play 4e, but when we did, we used this book, and liked it.
I also played plenty of Dragonlance in 4e... it is a strange setting, with lots of vanilla but also weird twists (like scarce metal, different magic, etc.). I cannot compare this 3.5e version with others, but it is probably the one I'd pick.
Planescape is another classic setting that you can get in PDF or print.
I might pick me some Ravenloft too, although I'm not sure wbout which one. I was looking for the 2e version, Domains of Dread, but I've heard that the 3.0 version (by Swords & Sorcery Studios/Arthaus/White Wolf) is even better. I'm unsure about the new 5e Ravenloft book, but I'm a big fan of Ravenloft (you already know that if you're following this blog...). There is also Realm of Terror (2e), the first boxed set for the Ravenloft campaign setting. Tim Brannan recommended this one to me. Maybe I should start with the original... Do you have any favorites? Let me know in the comments!
There are other settings on sale, including the 5e Wayfinder's Guide to Eberron and many others. I haven't read this one, but the Eberron stuff I did read was interesting.
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