The Walters Art Museum, in Baltimore, Maryland, contains a collection of about 1,000 illuminated manuscripts. Most of these were collected by Henry Walters, who gave his entire collection of art to the City of Baltimore. The manuscripts come from diverse cultures from around the world. Nearly all of them were collected for the beauty of their illuminations. With the help of grants from the National Endowment for the Humanities, and in a project that I directed from 2007 to 2012, The Walters is in the process of creating digital surrogates of all its manuscripts, and publishing them under a creative commons license. Most of the Islamic manuscripts have been digitized, and we are now in the process of digitizing the English, Dutch, German, Armenian, Byzantine and Ethiopian codices; Starting in 2013, The Walters will be digitizing its Flemish manuscripts.
You may well first stumble across them through Flickr: www.flickr.com/photos/medmss/. This is the Flickr page for the Department of Manuscripts at The Walters. The Walters has put up about 4,000 images up, and have had over 1,000,000 unique image views. These are hosted under a creative commons license. We are free to use them for any non-commercial purpose!
However, the source of the data is www.thedigitalwalters.org/. This data archive has been designed to be machine readable as well as human readable. It has images at resolutions high enough for any practical purpose, as well as more easily downloaded derivatives. The easiest way to access the data is via the online catalogue which presents the data in a variety of ways.
This project is ongoing.