Thousands of links exist that refer to King Arthur. Here are a few that I found to particularly interesting, entertaining or useful to the game.
The Domesday Book Online
The Domsday Book was a survey of England done by King William the Conqueror, and is not only one of the earliest extant listings of medieval society, but the basis for much of the setting for Pendragon. Not for the economics, which I’ve generalized, but certainly for the places. Need the name for another manor in Salisbury (i.e.-Wiltshire)? This is the place to look.
English Heritage
This is a set of short movies that illustrates many medieval (and ancient) ways of doing things, such as this one, "How to Arm a 14th Century Knight."
Mysterious Britain
Need more strange places and phenomena or your campaign? Check this site.
By the way, to have been included in my descriptions of the Pendragon countryside the miraculous, marvelous and bewildering events of the supernatural must have been known during the Middle Ages, or may have been discovered afterwards, but are traced (either actually or legendarily) to then. Thus many ghost stories, for instance, do now qualify.
Hypertext Medieval Glossary
A lot of places have glossaries, but this on seems to be more complete than most others.
Free heraldry Clipart
About a million or so designs that you can use for your coat of arms.
Measurement in the Middle Ages
Ever wonder what a gill is, or a Scotch Troy Ounce, or a tun? And lots and lots more.