Chronicling America https://chroniclingamerica.loc.gov/
Library of Congress digitization project of regional newspapers. Deepest coverage is late 19th century and early 20th century.
Veridian https://veridiansoftware.com
Veridian hosts many different projects digitizing newspapers from all over the world
Google News Archive http://news.google.com
Selected archive of full-page newspaper images.
New York State Historic Newspapers https://nyshistoricnewspapers.org/
Wide range of newspapers chosen to reflect New York's unique history.
*Use your local public library's online newspaper database subscriptions for the following websites:
Early American newspapers, 1690-1876 Available through a local public library
Full text. Concentrating primarily on those papers that began publication before 1820.
ProQuest historical newspapers Available through a local public library
Full-text and full-image articles for newspapers. Ongoing project. The New York Times, 1851-2001 -- The Wall Street Journal, 1889-1987 -- The Washington Post, 1877-1988.
Access Newspaper Archive Available through a local public library
Contains tens of millions of searchable newspaper pages, dating as far back as the 1700s. Especially good for regional newspapers.
Historical newspapers online Available through a local public library
Indexes only. Palmer's index to The Times [London], the Official index to The [London] Times.