Both The Getty and The Metropolitan Museum of Art are releasing digital books (.pdfs) of their own publications about their collections as a “virtual library.” These books are complete .pdf versions of the print bersions, and are free to download.
These are high-resolution, reproduction-quality images with embedded metadata, some over 100 megabytes in size. You can browse the images, or look for individual “download” links on the Getty Museum’s collection pages. Before the download actually begins, the Getty site asks simple questions about how you plan to use the images.
The Getty released images of many of its most famous works, including paintings like Vincent van Gogh’s Irises, but I’m especially excited by the medieval manuscripts (The Getty purchased the Ludwig collection, a huge collection of manuscripts rich in psalters and books of hours several years ago, and already had a solid collection, and they’ve added mss. since).