The Geoffrey Chaucer website:
Contains audio links and an excellent tutorial for reading and pronouncing Chaucer's Middle English.

The New Chaucer Society:

Voice of the Shuttle:
Contains links to every important medieval studies site on the web.

The Labyrinth:
Another good starting place for research in all fields of medieval studies.

Medieval History in the Movies:

Arthurian Film:
A bibliography of scholarly sources and movie reviews as well as synopses of the films.

The Camelot Project (University of Rochester): 
A database of of Arthurian texts, images, bibliographies, and basic information.

The Englisc, an Old English listserv:
Features Old English translations of modern English texts, including the Gettysburg Address and "Rudolph the Red-Nosed Reindeer"

Old English Pages:
The best starting point for study of Anglo-Saxon literature and language on the web.