The New York Metropolitan Museum of Art recently put online more than 360 catalogs for exhibitions that have been shown at the museum for the past several decades. The catalogs can be viewed through Google books or downloaded in pdf format from the Met's own site. While light on maps and cartography, the catalogs contain thousands of images of paintings, furniture, prints, ceramics and other pieces of art from the Middle Ages, Renaissance, and Baroque periods up to the modern day. While not strictly map related, someone seeking to research a particular culture or era for a map or project would likely find the catalogs helpful in terms of reference material, they are located at: The Metropolitan Museum of Art - Titles with full-text online