The webpage, Databases and E-Resources at the Library of Congress, History and Genealogy, http://www.loc.gov/rr/ElectronicResources/subjects.php?subjectID=13, is a great list of links to various databases found at libraries and archives that can be beneficial to your research. You will find newspapers, documents, and finding aids.
While some of the links are locked and can only be used at the Library of Congress, there are many that you can access from home. Links include:
A2A-Access to Archives: English archival information from the 900's to the present.
Africa Research Central: A clearinghouse of African Primary Sources
Alaska and Polar Periodical Index
Archives de France
Australian Historic Records Register
Belgian Newspapers Online
Black Americans in Congress
Brooklyn Daily Eagle 1841-1902
Chronicling America: Historic American Newspapers
...and much more