Princeton's Index of Christian Art

http://ica.princeton.edu/

From the home page:

It gives me great pleasure to welcome you to the Index of Christian Art web site. Little did the founder of the Index, Charles Rufus Morey, realize in 1917 how this resource would develop to be the most important archive of medieval art in existence and the most specialized resource for the iconographer. The Index has developed a life of its own over the last ninety years which has extended beyond the confines of its home in Princeton and is destined to have an even greater impact with the development of an electronic resource such as this. Despite the initial belief that all medieval art could be iconographically analyzed within fifteen years of the start of the project, it is a task which is not only still ongoing, but one which is thriving and ensures a prosperous future for the archive.