Important Print Guides to Chaucer
The Cambridge Companion to Chaucer by 
The Oxford Companion to Chaucer by 
Sources and Analogues of the Canterbury Tales by One-stop shopping for Chaucerian scholarship
Selected Chaucerian Texts Online
Canterbury Tales (ed. F. N. Robinson, 1957)
Canterbury Tales (selected tales from the Hengwrt MS, ed. Paul G. Ruggiers, 1979)
Canterbury Tales (selected tales in modernized spelling, ed. Michael Murphy)
Boece (ed. Richard Morris, 1868)
Book of the Duchess (ed. W. W. Skeat, 1899)
House of Fame (ed. W. W. Skeat, 1899)
Parliament of Fowles (ed. W. W. Skeat, 1900)
Treatise on the Astrolabe (ed. W. W. Skeat, 1872)
Troilus& Criseyde (ed. B. A. Windeatt, 1984)
Troilus & Criseyde (ed. Michael Murphy, modernized spelling, abridged)
"Lak of Stedfastnesse" (ed. F. N. Robinson, 1957)
"Mercilesse Beaute" (ed. F. N. Robinson, 1957)
Other Chaucerian Resources Online
Chaucer MetaPage - A collaborative effort to provide a guide to on-line sources for Chaucer studies
The Electronic Canterbury Tales - A treasure trove of useful information about Chaucer and the Tales
Essential Chaucer - Selective, annotated bibliography of Chaucer studies from 1900-1984
Harvard University's Chaucer Pages - Comprehensive resource on the Chaucer and his world
The Open Access Companion to the Canterbury Tales - volume of introductory chapters for first-time, university-level readers of Geoffrey Chaucer’s The Canterbury Tales. The chapters have been created and edited by professional scholars of Chaucer, and all material is released open access and free of charge for classroom, scholarly, and personal use.
TMR Reviews of Recent Books about Chaucer - Compiled by Edwin Duncan, Towson University
Visualizing Chaucer - Resource on postmedieval illustrated versions of Chaucer
Middle English Compendium: 3 resources in 1: The Middle English Dictionary, HyperBibliography of Middle English prose and verse, and an associated network of electronic resources, including a large collection of Middle English texts.
Database of Middle English Romance - Comprehensive resource on verse romance, searchable by manuscript, keyword & verse form
Online Journals, Projects & Resources
Arthuriana: The Journal of Arthurian Studies
Bulletin of the International Arthurian Society & Journal of the International Arthurian Society
The Cotton Nero A.x. Project (Sir Gawain and the Green Knight)
The Dictionnaire Electronique de Chretien de Troyes (in French & English)
Important Print Guides to the Arthurian Legend
Medieval Arthurian Literature by 
The Cambridge Companion to the Arthurian Legend by 
The Oxford Guide to Arthurian Literature and Legend by Learning Old English
Peter Baker's Old English Aerobics
University of Glasgow's Essentials of Old English (also available as a free iOS/Android app)
University of Cambridge OE Grammar Exercises
Online Resources & Projects
Electronic Beowulf - Once a paid subscription, the EB is now free online! Includes a bibliography of recent scholarship 1990-2012
Resources for the Study of Beowulf - Great compilation of Beowulf-related information, sites, and projects
Old English Literature: A Hypertext Course Pack - An older site, but contains a number of hypertext editions of important OE texts
Fontes Anglo-Saxonic - Searchable, full-text versions of written sources used by Anglo-Saxon authors
Anglo Saxon Aloud - No longer active, but includes audio files of all poems written in Old English