Artsedge (activities and helps for educators to teach about the arts in all subject areas)
http://artsedge.kennedy-center.org/

Art History Resources
http://witcombe.sbc.edu/ARTHLinks.html

Artcyclopedia
http://www.artcyclopedia.com/index.html

Digital Art Museum
http://www.dam.org

Art Museums around the world and other Art Information
http://ucblibraries.colorado.edu/art/onlineresources.htm

Music Hall (from Copernicus, a good jump off for music resources and information)
http://www.edgate.com/musichall/educator/index.html

Classical Music
http://www.classical.net/

Essentials of Music (classical)
http://www.essentialsofmusic.com