British Library - Resources for Researchers
British Library - Turning the Pages
British Library - Turning the pages 2.0
Codex Manese - Zurich 1300 - 1340
Decretals of Gregory IX with glossa ordinaria (the 'Smithfield Decretals')
Digital Image Archive of Medieval Music
Digital Scriptorium - growing image database of medieval and renaissance manuscripts that unites scattered resources from many institutions into an international tool for teaching and scholarly research. It bridges the gap between a diverse user community and the limited resources of libraries by means of sample imaging and extensive rather than intensive cataloging.
Leaves of Gold - Treasures of Manuscript Illumination from Philadelphia Collections is a project of the Philadelphia Museum of Art and the Philadelphia Area Consortium of Special Collections Libraries.
Science Manuscripts - British Library - A Selection of manuscripts from pre-1600 on various subjects of science
The British Library Catalogue of Illuminated Manuscripts
The Medieval Bestiary
The National Library of the Netherlands Illuminated Manuscripts
Virtual Library of Medieval Manuscripts - Bibliotheque Virtuelle des Manuscrits Medieveaux - In French
Wellcome Image Library - More than 100,000 images. Some modern some medieval.