Probably one of the most comprehensive site on the internet concerning Arthurian texts and contemperary articles about Arthur.

THe Library of the University of Rochester has done a great job with this site and modeled it after The Perseus Project put on by the University of Tufts.

Of course, one of my favorite sites for Arthurian and Celtic texts is The Internet Sacred Text Archive because it includes many full texts, such as The Mabinogion translated by Lady Charlotte Guest and From Ritual to Romance by Jesse L. Weston.