General
Art History Resources on the Web
Artcyclopedia - Search art museums worldwide!
Impressionists - From Biography.com, there is information on Degas, Monet, Renoir and more!
Metropolitan Museum of Art - Timeline of art history.
Native American Art Resources  - Links to information about Native American Art, including symbols.
Surrealism: The Art of Self Discovery
Women Artists in History - If you need to do research on female artists, this site, organized by time period is the place to start!

Museums and Galleries
Art Museum - like a real visit to a museum!
Guggenheim Museum
Louvre Museum - Don't forget to choose English (unless you speak French!), and take a virtual tour of the museum. 
M.C. Escher Gallery - Take a look at some of the most well-known Eschers!
Museum of Modern Art
Museum of Web Art (MOWA) - What a cool place to visit!  This site makes the most of the Internet - you will need plug-ins to view some of the exhibits.
World Wide Art Resources - Use the searchable database to find the artist or museum you're looking for.

Artists
Ansel Adams at 100 - From the San Francisco Museum of Modern Art
Ansel Adams Gallery - You can find biographical information here.
Ansel Adams - A companion Web site to the PBS documentary film.
Ansel Adams - Photographs of the Japanese-American internment at Manzanar.
Exploring Leonardo - This site, about Leonardo da Vinci is created by the Boston Museum of Science.
Leonardo da Vinci - Find a lot of information about the artist and his background.
Leonardo's Codex Leicester - Leonardo's contribution to science.
Leonardo da Vinci - All about the Mona Lisa
Leonardo - From the National Museum of Science and Technology
Leonardo da Vinci: Master Draftsman