Chronicling America-archives from 1836 to 1922
Google Newspapers-comprehensive list of international newspapers
International Coalition on Newspapers-listing of newspapers across the globe
Newspaper Cat-browse by topic
Newspaper Digitization Projects-U.S. sources, many local or regional sources; some require subscriptions
Newspapers Source Plus-use EBSCO to search newspapers dating back to 1985.
The New York Times Replica edition-use ID 200331767