The Greg Stafford Pendragon Page
A great source of essays and unpublished game material.
The Nocturnal Media Forums
A pretty active community of very friendly, helpful people. I've been known to drop in from time to time to weigh in on weighty topics.
Pendragon Librus Wikipedius
The official Pendragon wiki, hosted by Nocturnal Media.
In the Wild
King Arthur Pendragon on Wikipedia.
A good, general overview of the game.
Le Mort D'Arthur on Project Gutenberg
The full text of the classic by Sir Thomas Malory.
A site with some amazing browser-based tools. A must-have resource for Gamemasters who need a quick NPC, coat of arms or Cymric name.