Database at Columbia containing brief bibliographic information and selected images for medieval and Renaissance manuscripts from multiple libraries in the US (including 29 manuscripts from Penn's Rare Book & Manuscript Library). The images are not just of illuminations but also of text pages and codicological and textual features such as rulings, prickings, and catchwords.
Website for the 2001 exhibit at the Philadelphia Museum of Art of manuscripts from the collections of the members of the Philadelphia Area Consortium of Special Collections Libraries, including Penn's Rare Book & Manuscript Library. Penn displayed a 13th-century French Bible (Ms. Codex 236) and a 15th-century nuns' prayerbook (Ms. Codex 141).
Johns Hopkins and the Morgan Library are collaborating on a site presenting manuscripts of the Roman de la Rose. So far the site includes six manuscripts, one each from the Morgan, Walters, and Getty, and three from the Bodleian, but there are plans for more (including Penn's own Ms. Codex 906). At the moment the images are password-protected, but there's a link to send an e-mail to request a password. There are also codicological descriptions of the manuscripts on the site.
The Pierpont Morgan Library is working with the Index of Christian Art at Princeton to place images from their illuminated manuscripts in their online catalog, CORSAIR. So far images are available from over 220 manuscripts (and you can see the current count of manuscripts and images at the top of the list of manuscripts with images in CORSAIR). A very nice feature of their interface is the ability to pull up lists of manuscripts selected by manuscript type, country of creation, and/or century of creation. To get to this option, follow the link on the medieval/Renaissance image entry page for "Sampler of medieval images" and from the upper right corner of the sampler page, "Browse lists of digitized manuscripts." From the entry page you can also go directly into CORSAIR and search by keyword or title browse with a limit to Medieval Images only, but as the titles are in the format "Fol. 031r, Psalm 027 (Vulg., 026) : initial D," unless you want to see all the first leaves photographed so far, the keyword search is likely to be more useful.
To view manuscripts through this site, select "Handschriften" and click "Auswahl," which leads to a list of collections, including manuscripts grouped by language -- Latin, Greek, Hebrew, and German. Also on the same page is a link to secondary literature on manuscripts and rare books at the Bayerische Staatsbibliothek, under the name Forschungsdokumentation zu Handschriften. The Latin and Greek manuscripts seem to have been photographed in black and white, but at least some of the German manuscripts are in color. There are links from the images into the BSB-Katalog.
Images of complete medieval manuscripts from the Abbey Library of St. Gallen in Switzerland. Detailed descriptions of the manuscripts are in German, but brief descriptions are available in English, French, and Italian as well.