This is a cooperative effort among inidividual states and the federal government to locate, catalog, and preserve on microfilm, U.S. newspapers from the 18th century to the present. A map links from each state to the organization within that state that is
This page highlights and links to digitization projects of historic newspapers including international and U.S. collections. The focus is on digital conversion efforts of historical newspapers rather than full-text collections of current newspapers.
This table provided by the University of Pennsylvania is an excellent starting point for online historical newspaper research. Some of the links are to subscription databases, but many are freely available.