The Sierra Club maintains a "John Muir Exhibit" containing a short biography, a lengthy list of books and articles by and about Muir, links to relevant photography collections, and lesson plans for teachers.
The National Humanities Center has constructed a site entitled "Nature Transformed: The Environment in American History." Short essays address a number of topics including "The Roots of Preservation: Emerson, Thoreau, and the Hudson River School," and "The Challenge of the Arid West."
History Now, the online journal of the Gilder Lehrman Institute, devoted an edition to the American West in 2006. One of the essays discusses the often neglected "women of the west."
The National Archives has posted a rich collection of images at this site.
The Library of Congress makes a searchable database of images available here.