The Art Institute of Chicago

The Art Institute of Chicago has an online collection featuring over 33,000 of the works in their gallery. While they do not address the reasons for selecting the objects they have for digitization, I would assume they selected the best items in their collection.
The collection can be browsed by genre or searched by keyword, artist, collection, accession number, title or type. In addition, each object has a "browse similar" box below it, in which it lists categories that might also be of interest to the viewer. Each individual item is a small photo, but most enlarge to full screen images when a user clicks on them.
Each genre has an about page that contains notes about the scope and content of the collection and each object contains metadata about that particular object. Metadata includes: title, date, measurements, information about the artist and the collection it is a part of, who the item was a gift of, and the medium used. The metadata varies between items but this is mostly because of the different types of art represented. Metadata does not include any information about the digital representations of the objects.
While a knowledge of art is not necessary to browse the collection, the site does not provide any extra help in explaining the different kinds of art. It also does not provide historical context or information about particular artists. Because of this, the site's intended audience seems to be regular museum visitors and perhaps people who would live too far from the museum to visit its physical location. Though it could be used by researchers, it does not appear that they are the intended audience of this particular collection.