Good topic!
My campaign is, like D&D itself, a blend of different flavours. The central area where most of the current action takes place is mostly based on the Lankhmar tales of Leiber, with a strong influence of "1st edition feel" (what can I say? I like dungeons.)
The rest is more exotic: the southern lands of Ninn is Babylonian/Assirian, and it is ruled by god-kings. It is constantly at war with a hidden and ancient realm from another dimension, that established itself in a jungle. The war has been going on for centuries, with divine intervention being common on both sides. That certain empire is Tsolyánu, who see this iron-rich world as a grand place for conquest.

This is also the place where I put the lost and ancient gods for the Conan/Solomon Kane-inspired adventures.
To the east were once the many cities of a highly advanced technical civilization. They were destroyed millenia ago, before current human reckoning. Nothing lives there, only evil spirits: the intense radiation left from nuclear bombardment makes the inner lands completely uninhabitable - and just by entering, one is doomed to a certain and painful death. Many try, for the glassified sands and the crumbled buildings are rumored to be very rich in rare and never before seen magic items and relics. Life remains on the western coast of this huge land mass: a few paranoid and ancient cities, who still dream of their former greatness. Once the first outposts of the men fleeing from the holocaust, they are now full of intricate customs, arbitrary and often delicately cruel laws, etc.
The flavour in this case is all the weird stuff I love - Vance, Clark Ashton Smith and the Arabian Nights. There is a lot of adventuring to do here - ruins all around, and of course there are the accursed barbarians who live on the edge of the wastelands - they hate and envy everything who doesn't share their misery. They even have a pitiful "king" of sorts, who rules a pathetic tent-town with amazing cruelty. He dreams of conquest and these are just being fulfilled - Sfanomoe, glass-domed jewel of the south has already been sacked and raped by his horde, and the others will surely follow in the downfall. (The observant will notice that these are the mabden from Moorcock)
There is also a huge ice-covered island/microcontinent to the northwest, but I don't know yet what is there, apart from more ancient cities and Cthulhu-worshippers.
