Low Magic Casters: Balancing spellcasters for a low-magic campaign.
Medieval Demographics: Medieval Demographics Made Easy, an article by S. John Ross.
A Boke of Gode Cookery: James Matterer's collection of medieval recipes which he has translated and adapted for the modern cook.
Blue Moon Aurora: This site has some well-constructed adventures, advice on duet roleplaying, and some roleplaying tips.
Medieval Demographics Made Easy: Numbers for Fantasy Worlds.
The Campaign Builders' Guild Useful forums for campaign building, with a play-by-post section.
The FidoNet AD&D Compendium Version 2.1: A collection of classes, monsters, spells, magic items and whatnot else for 2nd edition, created by the FidoNet community.
Encyclopedia Mythica, a thorough reference on real-world myths and legends. This is very useful for giving your campaign world a unique feel. Don't like the standard D&D goblins or kobolds? Look them up in the encyclopedia and see what you find. Wikipedia is also a good reference for this.