Perhaps the most complete U.S. historical digital newspaper guide is from the University of Pennsylvania.
Wikipedia maintains a list of online newspaper archives found in the US and abroad.
The University of Illinois has an excellent subject guide to online national and international newspapers.
An extensive list of national, international, and college newspapers is available from XooxleAnswers.
ICON, the International Coalition on Newspapers, maintains an excellent guide to paid and free newspaper digitization projects.
British newspapers online, 1901-1950, lists free and paid online newspapers from England, Scotland, Wales, Northern Ireland, and Ireland.
The first newspaper in the United States was printed in 1690. Since then, newspapers have been an important source of information about American events.
Because of their value, many newspapers are being made available in digital form. This usually involves scanning either the paper originals or microfilm copies and making those scanned copies accessible. (Access to contemporary newspapers is usually via the electronic files used to create the newspapers and/or web sites.)
The newspapers listed here may run up to the present in date, but the emphasis is on material prior to 1995.
Rachel Leow, a graduate student in Malaysian history at Cambridge University (in 2009), has some good general suggestions on working with newspapers as historical sources.