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).
Metropolitan Museum of Art
Metropolitan Museum of Art: Peach Blossoms?villiers Le Bel
From the Metropolitan Museum of Art the painting "Peach Blossoms?Villiers-le-Bel" by the American impressionist painter Childe Hassam is profiled with a large image of the piece, a description, and links to related topics and artworks.
Mark Harden's Artchive
The Artchive: Post Impressionism
An except from "The Bullfinch Guide to Art History," which explains how the term "post-impressionism" was first used and what is meant by it.