The Internet Medieval Sourcebook English translations of medieval texts.
Otfried Lieberknecht's Dante Site A remarkably complex page compiled by this German scholar
Labyrinth The Mother of All Medieval Web Sites. Run by Georgetown University: the first site to try to organize the mass of medieval material on the web and still one of the best.
Netserf An alternative to Labyrinth as a collection of web resources.
The "Online Reference Book" for Medieval Studies An alternative to "Labyrinth," another remarkably vast collection of material.
Dante Alighieri on the Web Includes translations, among them some of the lyric poems.
The Web Gallery of Art Sponsored by the Hungarian Academy of Arts and Sciences, this is a remarkably extensive collection of images of European art from the Middle Ages through the Eighteenth Century.
The Dartmouth Dante Project Wonderful scholarly resource, but you need some foreign language skills to make full use of it. Texts of every commentary on the Commedia from the 13th to the 21st centuries, in all languages, accessible on a line-by-line basis.