Digital Public Library of America: primary sources in many formats documenting the history of the United States, contributed by libraries, museums, and archives across the country.
Europeana: multi-lingual online collection of millions of digitized items from European museums, libraries, archives and multi-media collections
Gorky's Granddaughter: a documentary art project with videos and recordings of studio visits and interviews with artists
Documents of 20th Century Latin American and Latino Art, Museum of Fine Arts, Houston: a trove of publications, manifestos, primary sources, and archival records documenting 20th century Latino and Latin American art.
Metropolitan Museum of Art, Heilbrunn Timeline of Art History: the Met's comprehensive art history timeline
MoMA Learning: tools for researching and understanding Modern Art including: a glossary of art terms; definitions for themes (or movements) in Modern Art; links related articles and manifestos
The Printed Picture: An exhibition and publication documenting and explaining the history of the photographic printing process, by Richard "Chip" Benson, master photographer, master printer, and former Dean of the Yale School of Art
Smarthistory: free and open resource with essays, images, and videos by art historians