A librarian at Bowling Green State University has created some truly excellent portals to digitized historical newspapers freely available on the web.
Historical U.S. Newspapers Online
The home page includes links to sites that provide access to newspapers from multiple states, the remaining links are organized by state.
Historical Canadian Newspapers Online
The home page includes links to National and multi-province papers, the rest are organized by province.
Historical Newspapers Online
The guide provides links to digitized newspapers from countries around the world.