Comprehensive Sites:

The Labyrinth
This is the major Internet resource for medieval studies. It contains collections of medieval texts and links to other sites of interest. A new web-database version with advanced search functions is available here.

Netserf
Newly revamped page has multi-category medieval links from Archaeology to Women.

The Internet Medieval Sourcebook
Contains primary and secondary sources for things medieval.  Part of ORB, the Online Reference Book for Medieval Studies.

World Wide Web Virtual Library/Medieval Europe

Primary Sources:

Online Medieval and Classical Library (OMACL)
At Berkeley, it has a collection of "some of the most important literary works of Classical and Medieval civilization" available in electronic form.

Medieval Texts
A good listing of online texts "from and about the Medieval Period."

New Advent Church Fathers
Essential background reading for medieval Christianity.

Bibliography of Medieval Works in English Translation
Especially useful for designing course syllabi.

Medieval Drama in Translation

Links for Medieval Theatre History and Early Music
Part of the Centre for Research in Early English Drama. Records of Early English Drama (REED).

Teaching Reference:

Medieval Academy of America.

Research Center
Listings of Associations and publications in the medieval field are available at the Netserf.

The Consortium for the Teaching of the Middle Ages
Studies in Medieval and Renaissance Teaching (SMART)
Two publications dedicated to pedagogy.

Special mention:

The Medieval Latin Dictionary from Danish Sources
The best place to look up Medieval Latin words online.