I discovered this simple website that is a good resource for identifying historical newspapers online: United States Online Historical Newspapers is found at https://sites.google.com/site/onlinenewspapersite/Home/usa . Its author has a blog you can subscribe to which has news about various newspaper digitization projects.
It is not comprehensive, but used together with other indexes, such as Historical Newspapers Online at http://gethelp.library.upenn.edu/guides/hist/onlinenewspapers.html , you might find digital newspapers online you did not know existed.
FYI, if you find that one of these indexes identifies a digital newspaper at GenealogyBank, this subscription site is available for free at your nearest LDS Family History Center.
Of course, don’t forget about the Library of Congress newspaper resource “Chronicling America” found at http://chroniclingamerica.loc.gov/ . You can search their digitized newspapers by clicking on “Search all digitized newspapers 1836-1922” or locate microfilmed newspapers by clicking on “Search U.S. Newspaper Directory, 1690 to Present” and then order a film through inter-library loan to your local library.