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).
The histories of Rome, England, and Middle-Earth
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.