See also relevant authors on the Major Authors page.
Notes on Middle English Romance, by Jonathan Glenn
Dr. Glenn, from the Universit of Central Arkansas, has put together this helpful introduction to Middle English romance's major structures and sub-types.
Hosted by the University of York, this database indexes all Middle English verse romances, provides plot summaries, and indexes major themes/motifs across the corpus.
The Camelot Project at the University of Rochester
An older but still useful website of All Things Arthuriana. It includes plot summaries of the major Arthurian narratives and "biographies" of most characters, links to online texts, many many images, and (I think) some study guides. The site is richer in nineteenth-century Arthuriana than in medieval Arthurian stories, but all are useful. . . particularly if you are not yet familiar with the sprawling legends that have accreted around these characters.
Dr. Echard teaches Medieval Literature at the University of British Columbia, and these are the pages that she has designed for the courses she teaches there. They are particularly fruitful for Arthurian and Welsh (including the Arthur of the Welsh) information, some Chaucer, and the appearance of medieval narratives in print contexts (16th c. through to the 20th).
As the LOS page itself says,
The aim of this series is to establish a resource-bank of critical editions and/or translations of French texts. . . .The series accommodates editions in the original or in translation, or with parallel translation into English.
The selection includes many Old French lais and romances, most of which have been also translated into modern English.
This blog (out of University College London) is part of an ongoing scholarly project and offers good food for thought when dealing with the lai in Middle English or French.