There are thousands of newspapers available online that might be useful for your research. Depending on your topic, you might want to consult newspapers from countries other than the United States.
Historical Canadian Newspapers Online
Searchable full text for hundreds of historical newspapers published in Ohio between 1800-2010. Search or browse 19th and 20th century Ohio newspapers and view full-page pdf images.
Coverage: 1800-2010
The full text of hundreds of U.S. newspapers from more than 35 states published by African Americans, dating from 1827 to 1998.
Coverage: 1827-1898
The electronic version of of the popular magazine Harper's Weekly from the Civil War and Reconstruction eras, 1857-1871.
The entire contents of the New York Times from 1851 to three years ago.