"Sir Gawain and the Green Knight (ca 1375-1400)"
http://www.luminarium.org/medlit/gawain.htm
By Aniinna Jokinen: By far the most comprehensive site around, this nicely illustrated website provides a short summary of the poem, as well as links to the full text in Middle English and Fitts I and II in modern English. It also links to most other SGGK-related pages with summaries and interpretations of the poem, including excerpts from the Cambridge History of English and American Literature, and student projects from Pace University, the University of Texas, and the University of Pennsylvania.
"A Re-hearing of Sir Gawain and the Green Knight"
http://www.luc.edu/publications/medieval/vol2/hoffman.html
by Elizabeth Hoffman: A heavily text-dependent look at patterns within the poem, concentrating on the repetition of aural patterns and the significance of colors. A very structuralist approach.
"Gawain's Departure from the Peregrinatio"
http://www.luc.edu/publications/medieval/vol2/berger.html
by Sidney E. Berger: This essay considers Gawain's journey from Arthur's court and back as a variation on the archetypal pilgrimage, an important motif in medieval literature and society. With many references to the larger medieval sphere, this essay is a good cultural studies approach to the poem.
"The Poem as Green Girdle: Commercium in Sir Gawain and the Green Knight"
http://web.clas.ufl.edu/users/rashoaf/gawain/masterng.htm
By R. Allen Shoaf: Online version of a professional, published work that closely examines the poem's themes of exchange, the language of commerce and trade, and the seduction scene. And yes, what all this has to do with circumcision.
The Cambridge History of English and American Literature. "Vol.1: From the Beginnings to the Cycles of Romance"
http://www.bartleby.com/211/index.html
Section XV of this volume is written by the legendary Middle English scholar I. Gollancz, who also provides much information on the Gawain-poet's other works.
"A Psychoanalytic Approach to the Question of Masculinity in Sir Gawain and the Green Knight"
http://www.luminarium.org/medlit/masculin.htm
By Fred Griffiths: An interesting essay that concentrates on issues of sexual desire, masculinity, and medieval gender roles in the character of Gawain by applying Freudian psychoanalysis to the text.
The Camelot Project "Gawain: Texts, Images, Basic Information"
http://www.lib.rochester.edu/camelot/gawmenu.htm
A very basic database that focuses on the character of Gawain within a larger Arthurian context.