I have:
1) World Primer. A four-page summary of the world, including toplines on religion, races, history, as well as the basic philosophy of our group's game-play (influences for the campaign world, the basic structure of each session, power level, compliance with 3E/3.5 rules, magic level, etc.)
2) Campaign Tools. A section where I keep the Adventure Log, a list of NPCs by region, and supplements (letters the characters find, that kind of thing)
3) PC Builder. For building new characters. Information on the races and classes available, regional skills, religion, house rules for new feats, PrCs, Domains, Organizations (paladin orders, druidic orders, etc.)
4) Religions. A more in-depth study of religion.
5) Magic. Notes on the magical traditions of the world for each race and explanations of types of magic not in the PH (alchemy, blood magic, etc.)
6) "The Chronicles". A series of documents written in the voice of a sage of the world who was writing a history book. This is where the information on the geography and history of the world is found. It's all written as someone's personal journal and notes for a book he was going to write but never finished.
7) Contacts: Everyone's e-mail address. I had everyone make up a new Yahoo! address using their character name so that I could publicly post the address and it wouldn't screw up their normal e-mail addresses. It also makes it cleaner that way so they don't have to sift through 200 junk mails to get to the important stuff (D&D!). I also put links here to various useful sites.
8) Resources. A list of all of the books that can be used by players in the campaign to create characters (with my approval). I was also selfish and included a wish list of books I plan to buy in case my players (who are also my close friends) are thinkin' of buying me a birthday or Christmas gift.
9) Message Boards. A link.
10) Legal and OGL.
That's all.
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