Many of us already know of ProQuest, an online resource that offers many old editions of newspapers, especially the New York Times. Some public libraries make the ProQuest searchable database available on site, and this of course, is a free service. You may even be able to subscribe to the service for home use. You can learn more by visiting the ProQuest website at http://www.proquest.com/en-US/.
Over the last number of months, for those interested in old New York (City and upstate) newspapers, there's the Fulton History site (found at www.fultonhistory.com/Fulton.html)where you can "search over 10,258,000 old New York State historical newspaper pages."
I have recently discovered too that the Library of Congress has a searchable database, including old newspapers from twelve of these United States. The search engine can be found at http://chroniclingamerica.loc.gov/search/pages/.
All these searchable databases can be very useful for genealogical researchers, especially in conducting research of historical events, not only in the United States, but throughout the world.