Web Gallery of Art
The Web Gallery of Art is a virtual museum and searchable database of European painting and sculpture of the Gothic, Renaissance and Baroque periods (1200-1700). You can even take virtual tours such as one to the Cistine Chapel in the Vatican.
Watercolour Workshop
An online resource of watercolour techniques. There are sections on materials and instructions on how to use watercolours which include colour theory, planning washes and painting shadows, figures, skies and trees.