The Labyrinth: Resources for Medieval Studies is a major scholar site organized topically.
ORB: The Online Reference Book for Medieval Studies has besides the usual links a great deal of original scholarly material on a variety of subjects.
Paul Halsall's Internet Source Book is a marvellous gateway to historical documents and other useful materials.
The University of Calgary has a site that will be of interest to many readers: The End of Europe's Middle Ages: Fourteenth & Fifteenth Centuries.
Historical Materials on Knighthood and Chivalry Index