Making of America is an excellent resource for digitized journals, magazines and books primarily published during the Antebellum period.
The University Libraries are providing trials to several online digital archives of American 18th and 19th century documents and publications.
Acccess the Trial
Chronicling America: Historic American Newspapers
New York Times Historical (Access through the library link) IMLS Gateway to Digital Collections
CONTENTdm Collection of Collections
Open Archives Initiative OAIster
Digital Collections at Duke University
Chronicling America provides digital versions of selected U.S. newspapers published between 1836 and 1922.