This list of useful resources will continue to grow, so check back often.
American Memory, Library of Congress Digital Collections:
http://www.loc.gov/library/libarch-digital.html
WWW. History, George Mason University:
http://historymatters.gmu.edu/browse/wwwhistory/
Making of American Books, University of Michigan:
http://quod.lib.umich.edu/m/moagrp/
Documenting the American South, University of North Carolina:
http://docsouth.unc.edu/
The Avalon Project, Yale University:
http://avalon.law.yale.edu/subject_menus/major.asp
Washington State History, Washington State Government:
http://www.sos.wa.gov/history/
Washington State Historic Newspapers:
http://www.sos.wa.gov/history/newspapers.aspx
Washington State Classics of History:
http://www.sos.wa.gov/history/publications.aspx
The American Folklife Center: Library of Congress Digital Collections:
http://www.loc.gov/folklife/onlinecollections.html
Documents for the Study of American History, Virtual Library:
http://www.vlib.us/amdocs/index.html
Virtual Library History Central, European University Institute:
http://vlib.iue.it/history/index.html
Internet Archive: http://www.archive.org/
National Archive: http://www.archives.gov/research/arc/topics/
Nines: http://www.nines.org/
Columbia River Basin Ethnic History Archive:
http://archive.vancouver.wsu.edu/crbeha/home.htm
Black Oral History Collection, Washington State University:
http://content.libraries.wsu.edu/cdm/landingpage/collection/5985/
History Overview, US Census Bureau:
http://www.census.gov/history/www/through_the_decades/overview/ (See left column for census years, then page down to "Further Information" for links to specific census data.)
Office of the Historian, US Department of State: http://history.state.gov/
Declassification & Transparency, National Security Agency:
https://www.nsa.gov/news-features/declassified-documents/
The Vault, Federal Bureau of Investigation: https://vault.fbi.gov/