Dark Sun is really well done. There are no gods because they basically gave up and went away or the setting exists in a time-space pocket or something, so clerics worship elemental forces. Defilers are the common type of mage and their spells cause pollution and destruction of natural resources, causing the hellworld to be even more inhabitable. Metal is rare, so most gear is made of leather/bone/obsidian/etc.
This is a post by me, so I have to mention that there's a long-running MUD based on this setting (unofficially) called Armageddon which is notable for some troubling accusations. Human nature is such a shame.