Perhaps my favorite part of gaming is worldbuilding, that is, creating a plausible and vivid setting for characters to interact with.
Kosmogogues is an excellent collection of resources for worldbuilding.
This site links to pages that deal with building worlds in fantasy and science fiction. Included are links to software that can aid gamemasters in creating the realm of their dreams. (Compiled on AdventureGamer.Net).
How to Build A World. 'nuff said.
The World Building Homepage More for sci-fi than fantasy. Many links.
World-Building Email List, which includes a page of links.
Planetary Classification List also useful for sci-fi worldbuilding.
Links to star-map design
World Builders: This is a site for a class on world building taught in Los Angeles. Very intensive from what I saw, from microbiology to astronomy and everything in between.
for fantasy writers: Patricia C. Wrede's Worldbuilder Questions Once you have the basic ideas for your world, this site offers questions to get you thinking about details.
The Mythopoet's Manual: "Writing Multi-Cultural Fictional Settings 1st Draft, including many author's notes to self" by Loren J. Miller
J. Vesanto's World Building Page has some good links to sources all over the Net, from articles about worldbuilding and how terrain goes together, to links for ancient languages and swordplay. Check out his guide to developing magic systems.
It's my opinion that J.R.R. Tolkien was perhaps the greatest fantasy world builder. He gets a page of his own.
Klingon Language Institute deals with a fictional language that was actually developed by a linguist.
The Rosetta Project is dedicated to cataloging and preserving some 1,000 languages. They predict that 50-90% of these will be extinct in the next century.
Here are a number of sites of particular interest for building a setting based on the European Middle Ages.
Medieval Demographics Made Easy A fine site for population numbers, including members of different professions. The article includes links to population calculators.
An article on Cities, Population, Subsistence and Land Area, from the CityDesign project (a worldbuilding project based on the Rolemaster gaming system).
Andrew's Harnic Manor Generator. It's geared towards Harn (which is one of the richest FRPG worlds in my opinion), but has applications for quick and dirty estate creation.
If you're interested in authentic names for characters from various countries in a medieval setting, the Academy Library is a good place to look. It's also useful for heraldry information.
Medieval Names Archive
The First Thousand Years of British Names
Anglo Saxon Names compiled from Bede's A History of the English Church and People
The Web is full of resources a gamemaster can use to flesh out a world. This site is full of "real-world" links that may prove useful. I've used most of these in the course of fiction writing or game development.
A gamemaster is the eyes and ears of the characters. Players will ask you what things look like. Don't know how a house is laid out? There are house plans galore at Cool House Plans. For plans of all kinds, from mansions to galleons to strip clubs, try Deckplans, Floorplans And Assorted Diagrams and More Deckplans, Floorplans And AssortedDiagrams .
Want to know about the GNP of Switzerland, the government system of New Zealand, or the size of the standing army of Ukraine? Check out the CIA's World Fact Book for 1999. And here's a trick: if you want to know the facts from previous years, change the URL from "wofac99" to "wofac97" or whatever year (it goes back at least to 96).
INTERPOL doesn't have field agents like the CIA, FBI, or Scotland Yard. Rather, it is an international agency that coordinates information among law enforcement agencies. Still, the organization has proven useful in my games. (Note: this link has been temperamental, but if I find a better one I'll update it).
since February 27, 2000