The WebMuseum, Paris. Select the period in art history you want.
Web Gallery of Art: Artists are listed alphabetically, so you need to know who you're looking for. Might want to turn your volume down...
Chris Whitcombe's Art History Resources on the Web. Art & artists are listed by time period.
Mark Harden's Artchive. Listed both by name and period.
Artcyclopedia The Fine Art Search Engine links to external sites like museums where works of art can be found.
The clickable School of Athens
Web Gallery of Art - Visit to the Sistine Chapel
Virtual Sistine Chapel - from the Vatican web site
Don't miss the Folger Shakespeare Library