Digital image collection of hundreds of historic newspapers. Coverage is 1690s-1922; issues available vary by title. You can search the entire collection or limit to a publication. Part of the World Newspaper Archive collection.
A collection of digitized historic American newspapers, 1789-1963. Freely available.
Also has "U.S. Newspaper Directory 1690 to present": a list of all known newspapers, their publishing histories, library locations and holdings. Select "Help" for information about the newspaper collection and for Search Tips. Select "Recommended Topics" for examples of interesting searches.
The University of Delaware Library has scanned selected newspapers and added them to Chronicling America. See the Delaware list.