Web Gallery of [European] Art
This is a great site of European art, with something like 13,000 works of various media.
In particular I found the Medieval Section to be fascinating, especially the illuminated manuscripts.
Also of interest are sections on "unknown artists", for example the unknown Flemish Masters. Look for "Masters" in the M section.
Finally, I spent some time in the in depth sections on Michelangelo and Bernini.
Tags: art Michelangelo Bernini Medieval