Les très riches heures du Duc de Berry
Images from a very famous 15th century book of hours.
The Age of King Charles V (1338-1380)
1,000 Illuminations from the Department of Manuscripts at the Bibliothèque Nationale de France.
Bodleian Library (Oxford) Digital Library Projects
Manuscripts and images from all timeperiods. Check out particularly the "Bodleian Library Broadside Ballads Project" and the "Early Manuscripts at Oxford University" sections.
Hill Monastic Manuscript Library
Det Arnamagnæanske Institut, Denmark
Currently only selected images from the collection of Scandinavian manuscripts are available here.
The Saint John's Bible
Not medieval, but a new project to create a handwritten and illuminated bible. Unfortunately, not much information on methods and materials, which is what I wanted to see.