A site "totally dedicated to the Early Celtic Kingdoms of the Isle of Britain and its diminutive counterpart across the Channel [Brittany]. You'll find lots of information here about the celebrated and not-so-celebrated Kingdoms that flourished in Britain from the time the Romans left until well into the 11th century."
In Search of Britain's Lost King: A Millenial Quest for Arthur
Created by two undergraduate students who traveled across Britain to explore sites related to King Arthur, this website is an incredible compilation of Arthurian history and the landmarks associated with Arthur.
This section of "Odin's Castle" is devoted to King Arthur links -- general history, the Grail, Geoffrey of Monmouth, Le Morte D'Arthur, Arthurian archaeology, Merlin, and more. It also has a section on Robin Hood.