Chronicling America: Historic American Newspapers (Library of Congress): Newspapers from select states are available from 1880-1922.
The New York Times Archive: Search this newspaper for the period 1851-1980.
Wyoming Newspaper Project: Contains newspapers printed in Wyoming from 1849-1922.
American Memory (Library of Congress)
McCracken Research Library Digital Collection (Buffalo Bill Historical Center)
National Archives & Records Administration (NARA)
Smithsonian Institution Research Information System: "Search over 2 million records, with 210,500 images, video and sound files from dozens of Smithsonian museums, archives, and libraries."