Times Machine

Over the last few years, several newspapers have made the expensive investment of scanning their microfilm to make stories available online that are older than their text archives, started in the early 1980s or later.
ProQuest has done several newspapers, as well as Olive, Heritage, and Cold North Wind. Some are available online, by subscription, or through the newspaper's Web site, downloaded for a small fee; some public libraries offer access to ProQuest papers, too.

The New York Times is one of those papers, and you can search and download individual stories from their older archives in PDF ProQuest format.

But now the Times has launched Times Machine, where you can actually browse scans of an entire newspaper. If you're a home delivery subscriber you can use the full service, covering years from 1851-1922. A few papers are available for non-subscribers, covering major events and '100 years ago today'. (Note the page also suggests checking if your local library has access.)

The Times' Open blog discusses the project, which uses Amazon Web Services. Video description via Scobleizer. The software is not perfect: on the first 1851 paper, in a story about Lancaster, PA the summary spells it Laucaster.