D&D, AD&D, AD&D 2e, and again, D&D. If it weren't for the game, I'd never have created the world.
Maxfield Parrish, artisan of many a fantastic landscape. The forest in The Enchanted Prince is the forest at the base of the Worldwall (frog prince & woman optional).
Jon Anderson's Olias of Sunhillow and Yes' Close to the Edge & Fragile. And Relayer. Ah, heck just about any Roger Dean artwork.
The histories of Rome, England, and Middle-Earth
A sprinkling of The Sword and the Sorcerer
A very tiny mote of Stephen Donaldson's Chronicles of Thomas Covenant
It's not really a source of inspiration, but City is just about the most useful tool a world designer would ever need.