Many have gone before me in creating indices of web sites, I make no claim to ahving a complete listing, therefore, I include the works of others and attempt not to overlap too much.
Medieval Source Book - Fordham University new York, New York
Medieval Links - private, excellent collection arranged by topic.
Digital Librarian - the end all and be all of internet sources.
Online Museums of the World. Takes a little work but it allows you to view collections on line of museums from all over the world.