This is going to be a short one, I’m afraid, but who says a blog entry has to be long to be useful? The Camelot Project is available thanks to the University of Rochester, and it is amazing. I first ran across it while I was writing The Prometheus Tapestry; I was looking for source material on Gawaine other than the Green Knight poem. The collection is not exhaustive, of course, but it has a lot of useful texts and artwork online (the reference sections are not thorough built out at this point).
The quality varies widely, but there’s plenty of good stuff, and a lot of writers who were unfamiliar to me. Other than the wealth of poetry and stories from centuries past (some of which are usefully footnoted), of particular delight is Raymond Thompson’s series of interviews with late-20th-century authors who have written in the Arthurian world. There sure are a lot of us. 🙂