What are some good online collections of artwork?
One site I’ll recommend here is the Art Renewal Centre which is one of the largest online collections I’ve seen while being largely limited to stuff I like: http://www.artrenewal.org/
Web Gallery of Art is definitely worth checking out.
It covers the time period from the medieval ages to realism in the 19th century.
The Public Catalogue Foundation and the BBC have now put online images of every painting in all public collections in the UK.