The Morgan Library Black Hours, one of the world's most beautiful and striking illuminated manuscripts, has been digitized in its entirety. Richly decorated in blue and gold on black vellum, it is one of a surviving handful of such manuscripts produced in late 15th century Bruges. (Poorer quality, but still interesting, images of another such work, the Black Hours of Charles the Bold, are also online.
Visitors to the Morgan Library in New York will have a rare opportunity to view one of the great masterworks of medieval illumination, the Hours of Catherine of Cleves. But if you don't have a chance to visit, all 157 miniatures have been digitized.