Dark Sun

Dark Sun was a post-apocalyptic Dungeons and Dragons world, if there ever was one.  Magic run rampant had destroyed the ecosystem, and those few sentient beings that remained eked out a meager living in desert dunes, hunting for water wherever it could be found.  Dwarves had no beards, elves were thieving con-artists, and halflings were rabid cannibals - this was not your father's D&D.  The magic system was set up brilliantly - you could either be a defiler or a preserver.  Defilers advanced in levels much more quickly, but drew their power from the earth itself, killing all plant life in the surrounding area each time they cast a spell.  Preservers take only as much life from the land as they need to cast the spell, nothing more, and as such take a much slower path to power.

Clerics are very different, as there are no gods in Athas, at least not the type that you'd be used to playing D&D.  The clerics there worship the elements themselves.

The two most precious commodities on Athas were metal and water.  Metal weapons are extremely rare, and most weapons were made of obsidian or bone.  Water is almost impossible to find under the scorching sun, and dehydration checks were a big part of any adventure that took the party outside of city walls.

The cities themselves were ruled by the Dragon Kings, fearsome defilers who had gained so much power that it had twisted and corrupted their bodies.  They had all been the servants of the inventor of magic, Rajaat, and each had overseen the genocide of an entire race.  Each had been awarded a city for their efforts, but turned on Rajaat and imprisoned him.  Each Dragon King is served by Templars, quasi-clerics who receive their magic directly from the beasts they worship.

Rampant slavery, seas made of silt, playable insect races (thri-kreen!), psionic powers everywhere, this was a world where all bets were off.  The 4e revision of Dark Sun was... alright, but if you want it the way it was meant to be, check out the 2e version.