AD&D 2e / D&D 4e Dark Sun: a post apocalyptic setting
A core premise of the Dark Sun setting was that an apocalypse happened, magic profoundly changed. Gods / deities and their connections to PCs were more or less removed, cleics draw from elemental planes rather than divine power, and it was as much a survival test as anything else. The state of Athas is degraded due to both technological and magical effects in the past. Arcane magic tends to be either "preserver" or "defiler" category.
Dark Sun differs further in that the game has no deities, arcane
magic is reviled for causing the planet's current ecological fragility, and psionics are extremely common.
There are a list of adventures/modules at the link in the title.
It was inspired by the "dying earth" genre of SF/Swords and Sorcery short stories and novels.
Dark Sun is an original Dungeons & Dragons (D&D) campaign setting set
in the fictional, post-apocalyptic desert world of Athas. Dark Sun
featured an innovative metaplot, influential art work, dark themes,
and a genre-bending take on traditional fantasy role-playing. The
product line began with the original Dark Sun Boxed Set released for
D&D's 2nd edition in 1991, originally ran until 1996, and was one
of TSR's most successful releases.
Athas is not the warm and cuddly Forgotten Realms, no, and it's not even the World of Greyhawk. It's just dangerous.
How available Dark Sun materials are may depend on where you are and what you are willing to spend, but I think I saw some at DriveThruRPG. There were a number of novels published for the Dark Sun setting that you may be able to pick up for cheap at a second hand book store. (I found them easy reading while I was at sea, but as with any TSR published novels, the quality varied).
You can also find Dark Sun materials from 4th edition, but I have no experience with that edition.
You may find it of interest that Jack Vance, whose magical system Gygax adapted for the original game, wrote a number of Dying Earth stories that are on the reading list of inspiration for D&D).
I only got to play this for a little while, now and again, due to game groups breaking up and me giving up the hobby for a while, but of all the settings that D&D had before, this is the one I am hoping WoTC will resurrect into a D&D 5e form. The devs are still trying to get the psionics thing to work (there have been 3 UAs so far) and it will or won't pan out. (Sorry, Mr Baker, Eberron is nice, but Dark Sun is IMO better in the sense of "less is more.")