18thConnect (18th Century Scholarship Online)

Archives of American Art (Smithsonian Institution)

The Artchive 

The Art Museum Image Consortium 

Art Project (Google)

BBC Archive

The British Museum Highlights

CalPhotos (U of California at Berkeley)

Central Connecticut State University Digital Collections

The Clearinghouse of Image Databases (University of Arizona)

Connecticut Digital Collections

Connecticut's Aerial Surveys (photos)

Digital Collections at Harvard University

Digital Collections in Colorado

Digital Image Catalog (Berkeley Digital Library)

Eternal Egypt

Europeana: think culture

FedFlix ("best movies of the United States Government")

The Fine Arts Museums of San Francisco 

Florence Griswold Museum (Lyme, CT)

The Frick Collection 

Hispanic Digital Collections (Library of Congress)

ImageBase (Virginia Tech)

Immigration to the United States, 1789-1930 (Harvard University) 

The John F. Kennedy Presidential Library and Museum Digital Archives

Kentuckiana Digital Library (search all)

Kentuckiana Digital Library Images (50,000)

The Louvre 

Metropolitan Museum of Art Collections 

Fundacio Joan Miro

Picasso (Online Picasso Project)

Museu Picasso

Museum of Modern Art - Exhibitions and the Collections 

NASA Images

National Library of Medicine Digital Collections (biomedical books and videos)

National Museum of African American History and Culture (Smithsonian)

New York Heritage Digital Collections

The New York Philharmonic Digital Archives

New York Public Library - Digital Gallery

New York Public Library - Small Town America 

New York Public Library - Images of African Americans from the 19th Century 

Nines (Networked Infrastructure for Nineteenth-Century Electronic Scholarship)

Norman B. Leventhal Map Center (Boston Public Library)

Photomuse

Open Access Journals

Scholars Resource (images for teaching art history) 

The September 11 Digital Archive

Smithsonian Collection (Smithsonian Institution)

Smithsonian Digital Library

The State Hermitage Museum 

Teaching and Learning Resources (Federal Resources for Educational Excellence)

Wadsworth Atheneum Collection Highlights 

Web Archive Collection Service (WAX) (Harvard University Library)

Web Museum - Paris 

Women Working, 1800 - 1930 (Harvard University)

World Digital Library

Yale Art Gallery - Collections 

Yale Digital Commons (search 1.5 million digital assets from across Yale's collections)