The Internet back in the day
Remington Rand Computer: Consolidated Convair Aircraft Factory, San Diego Equipment, 1950s, San Diego Air and Space Museum Archives (Flickr Creative Commons)
Museums
Museums, complied by the University of Michigan
Archives and Special Collections
The Archives of American Art, The Smithsonian Institutes research center housing the history of American art
Art and Architecture Collection, New York Public Library
Classical Art Research Center and The Beazley Archive, University of Oxford
Online Scholarly Resources
Medieval Art: A Resource for Educators, Metropolitan Museum of Art (download the PDF)
Medivalist.net: Where the Middle Ages Begin, Peter Konieczny (librarian from Toronto, Canada) and Sandra Sadowski (scholar from Toronto, Canada)
The Labyrinth, Resources for Medieval Studies, Georgetown University
The European Library, online portal to 48 national libraries in Europe
Dumbarton Oaks Research Library and Collections, an institute in Washington, D.C. that supports research in Byzantine studies
Dictionary of Art Historians, Department of Art, Art History and Visual Studies, Duke University
Catalogue of Digitized Medieval Manuscripts, UCLA Center of Medieval and Renaissance Studies
Medieval Manuscripts on the Web, Sian Echard, University of British Columbia
Online Medieval Sources Bibliography, Center for Medieval Studies, Fordham University
Other Catalogs to Search
Metropolitan Museum of Art Collections Database
Catalog of Illuminated Manuscripts, British Library
Warburg Institute, University of London
Bibliography of the History of Art (BHA) & the Repertoire de la Litterature de l'Art (RILA), covering the history of Western Art through Late Antiquity, 1975 - 2007, Getty Institute
Internet Sites
Google Images, a search engine for online images
ORB, The Online Reference Book for Medieval Studies
Art History Resources, Christopher Witcombe
e-catena, Early Christian Writings