A collection of Arthurian texts, images, bibliographies, and basic
information hosted by the University of Rochester. This is the place to start looking.
Tom Green has assembled not only a good collection of annotated scholarly Arthurian links, he's written several interesting and thoroughly cited introductiory essays on various Arthurian subjects, including a good essay on Myrddin.
This is a site by Jeff Davies that tries to put what we know or suspect about Arthur into an historical context. He uses both primary and secondary sources, and has lots of source references.
Arthuriana is a journal of Arthurian studies. This site has the table of contents of the current issue, and a searchable index of past issues, and a variety of other resources including pedagogical resources.
This is a collection of downloadable Middle English texts, including Malory's Morte, Sir Gawain and the Green Knight, related poems, and several other Arthurian texts. You can even search all the texts in the collection
The Charrette project is a scholarly, multi-media electronic archive contianing the text and images from the manuscripts of ChrŽtien de Troyes's Le Chevalier de la Charrette (Lancelot, ca. 1180). It also includes modern scholarly editions of the medieval French text. The project was developed and maintained by the Department of Romance Languages, Princeton University.
This extensive, categorized list of Arthurian fiction,poetry, and other works, hasn't been updated since 1996, but it's still very useful, and interesting.