The American State Papers can also be found online at:
American Memory - The American Memory Project from the Library of Congress provides free access to PDF images of the American State Papers volumes.
Hathitrust - A digital library with many U.S. government publications.
The Serial Set can also be found online at:
American Memory - The entire set has not been digitized, only portions from the 23rd to 64th Congress.
Congress.gov - 104th Congress (1995/1996) - Present. Full text of committee reports and some Senate treaty documents. Search by keyword, bill number, report number, committee name or sponsor.
Govinfo.gov - 104th (Congress (1995/1996) - Present. Includes full text of Serial Set publications in html or pdf format. Search by keyword, bill number, popular name, committee name or sponsor.