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 -- AICT is a personal, non-profit project of its author, art historian, and visual resources curator Allan T. Kohl. AICT is intended primarily to disseminate images of art and architectural works in the public domain on a free-access, free-use basis to all levels of the educational community, as well as to the public at large. The collection focuses on ancient, medieval, and Renaissance European art and architecture.
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 -- WGA is a virtual museum and searchable database of European painting and sculpture from 11th- to mid-19th centuries. Content focuses on the Renaissance period, the Medieval roots of the period, and its evolution to Baroque and Rococo via Mannerism. More recently Neoclassicism, Romanticism, and Realism were also included.