Source: Newspaperarchive.org

Heritage Microfilm has a fee-for-service website, http://www.newspaperarchive.org/. It proclaims that it

is the world’s largest historical newspaper archive online, with millions of newspaper articles from 1759 to the present. Every newspaper in our archive is fully searchable by keyword and date, making it easy for you to explore historical content and family stories.

Much of its database appears to be community and local newspapers. You can get a sense of its holdings by surfing to http://www.newspaperarchive.org/BrowseLocations.aspx?pages=30 and browsing for newspapers by state.

As described previously, the National Endowment for the Humanities (NEH) is developing a free database of digitized United States newspapers published between 1836 and 1922.