This is a list of free and pay wall blocked digital online newspaper archives. Most are scanned from microfilm into pdf, gif or similar graphic formats and many of the graphic archives have been indexed into searchable text databases utilizing optical character recognition (OCR) technology. Some newspapers do not allow access to the OCR-converted text until it is proofread. Older newspapers are still in image format, and newer newspapers are available as full text that can be cut and pasted. Most text is in ASCII, some are using Unicode for diacritical marks not available in ASCII. Google now indexes many newspaper archives.
This table provides a list of historical U.S. newspapers that are available online at no cost. Newspapers available for free through Google News Historical Archives and Newspaperarchives.com are listed individually as I identify them. Newspapers available through Chronicling America and state digitization projects are usually listed as a group. For instance, under "Wyoming" I have not listed every newspaper digitized in the project but simply described what is available.
Links to several dozen newspapers' sites, sortable by name and by city. A drop-down menu lets you choose which section of the paper the links go to, which is useful though it needs some updating.