National Gallery of Art
National Gallery of Art: Impressionism
The site from the National Gallery of Art contains a short overview of the Impressionist movement - several works are featured, with audio analysis of the paintings. More extensive galleries of Edouard Manet and Claude Monet are linked.
National Gallery of Art
National Gallery of Art: American Impressionists
This site provides many examples of American Impressionistic work. Also, under the section titled "Overview", it gives information into the origins of American Impressionism (click the "continued" link after the paragraph).