Amit Sood is the head of Art Project, a Google initiative to bring the world's art museums onto the web. Visit it at googleartproject.com.
American Memory Project -- Created by the Library of Congress, American Memory is a gateway to rich primary source materials relating to the history and culture of the United States. The site offers more than 7 million digital items from more than 100 historical collections.
Art Images for College Teaching
ArtProject.com -- A unique collaboration of over 150 acclaimed art museums in 40 countries. The search capabilities are very clunky and it is recommended to use the Search by Museum category to browse. Create and share your own "collections."
Visual Arts Data Service (VADS) -- A well-designed site with a focus on fashion, craft, 2D design, and contemporary artists working in the United Kingdom and internationally. It is a repository for many special collections in the UK.
Web Gallery of Art