ECHO European Cultural Heritage Online
ECHO, part of the Europeana digital library, provides access to nearly two million digital objects including film, photos, paintings, sounds, maps, manuscripts, books, newspapers, and other documents
Metropolitan Museum of Art
The largest art museum in the United States hosts a wonderful website with images, essays, timelines, and more.
MoMA Museum of Modern Art
A valuable resource for modern art history research.
NYPL Digital Gallery
New York Public Library's images digitized from primary sources, maps, prints, photographs, books and ephemera.