World-Design Tips: Things to think about when creating a game world.
Creating a Realistic Map: Use real-world geology and weather patterns to draw better maps.
City Design: Some useful (but very sparse) notes on city design.
City Features and Flavors: Tips on making realistic cities.
Medieval Maps: A collection of medieval maps of cities.
Creating a Religion: Tired of the contrived polytheistic religions found in role-playing games and written fantasy? Here's a long list of questions, to help you create a more realistic fantasy religion.
Mysticism and Shamanism: An article from Imazine Rolegaming Quarterly.
Strolen's Citadel A large collection of locations, NPCs, items and other things you might want to include in your campaign.
Errant Dreams: This site has articles on character creation and world building.
Donjon's Random Generators: Everything a DM could want, from NPCs to magic items to calendars to maps, this site can generate for you. Very handy. There are also DM tools such as a dice roller and an initiative tracker. There are generic generators, and generators for several different game systems, with an emphasis on D&D and Pathfinder.