Make sure you have Real Audio installed before you visit. National Museum of American Art Produced by the Smithsonian Institution, this site claims to have pictures and information on over 1000 works of art. Check out the White House collection of American crafts. If you teach musical concepts, especially music composition, this is a site you must visit. You can set up a class account which will allow you to put music lessons online and allow your student to test their composition skills.
Students can use the built in tools to experiment with transposition, tempos and other alterations.
You also have the option of uploading or downloading standard MIDI files from your account. The site is free at the moment but requires you to register in order to set up your class account. You will also need to download and install a special browser plug-in. Considering all the resources here, I won't argue with that. They feature a very comprehensive list of theatre-related sites, a directory of theatre professionals from all over the world and theatre listings from all over the world.