The National Gallery of Art, Washington, D.C. Just about everything you might want in a museum website.
Hermitage, St. Petersburg, Russia. This site includes 3D images of works and virtual tours of galleries and the exterior.
The Van Gogh Museum, Amsterdam, Netherlands. Great details of works by van Gogh. Biographical information on the artist.
J. Paul Getty Museum, Los Angeles, CA. See especially for art conservation.
Museums in Florence, Italy. This site offers links to the Uffizi, the Accademia, the Pitti, and other State Museums in Florence.