These are excellent resources for primary sources.
American Journeys http://www.americanjourneys.org/
American Memory http://memory.loc.gov/ammem/index.html
American Presidency Project at http://www.presidency.ucsb.edu/
Avalon Project: Documents in Law, History, and Diplomacy at http://avalon.law.yale.edu/subject_menus/major.asp
Born in Slavery: Slave Narratives from the Federal Writers’ Project, 1936-1938 at http://memory.loc.gov/ammem/snhtml/snhome.html
Colonial Williamsburg Digital Library http://research.history.org/DigitalLibrary.cfm
Documenting the American South http://docsouth.unc.edu/browse/collections.html
Google Books at http://books.google.com/
Internet Archive at http://archive.org/
Jesuit Relations at http://puffin.creighton.edu/jesuit/relations/
Making of America (Cornell University) http://digital.library.cornell.edu/m/moa/
Making of America (University of Michigan) http://quod.lib.umich.edu/m/moagrp/
National Archives http://www.archives.gov/
Project Gutenberg at http://www.gutenberg.org/
Voyages: The Transatlantic Slave Trade Database at http://slavevoyages.org/tast/index.faces
Women Working, 1800-1930 at http://ocp.hul.harvard.edu/ww/