The Louvre, Paris, France (official site)
Although you will only find a handful of images from this wonderful museum, the major works are on line. English, French, Portugese, Spanish.
The Uffizi, Florence, Italy (official site)
or this page which tells about the history of the museum.
The Tate Gallery, London, Great Britain (official site)
The Vatican Museum and the Sistine Chapel, The Vatican
The Vatican has some of the most incredible world treasures on-line, from ancient civilizations to present. This is a must-see.
Tchochkes available here.
A strange archive of photos from the museum are here.
The Metropolitan Museum of Art, USA (official site)
The Smithsonian Museum, Washington, D.C. (official site)
The Museum of Modern Art, USA (official site)
The Guggenheim Museum (official site)
Tokyo Fuji Art Museum
The San Jose State University Art Gallery, USA This includes a search engine to find cultural photographs from around the world.
World Art Treasures, Switzerland By the Jacques-Eduard Berger Foundation. This includes over 100, 000 images.
Other Museum Listings
With mirror sites all over the world, this hosts a great deal of famous paintings, currently they have a special section in medieval art and Paul Cezanne.
The World Arts Resources list of all the on-line museums
Museums -- Exhibits and Special Collections
Einet's listings of museums
If you cannot find a work of art that is in one of the above museums, fear not, that doesn't mean it's not on-line somewhere in the vast world of the internet. Try the sites below:
The Perseus Tuft Project
Ancient Art and Architecture from Tuft University's archives. This includes an excellent search engine.
"Our mission is to become the definitive and most effective guide to museum-quality fine art on the Internet."