How far back do your newspaper's online archives go? The U.S. Library of Congress has a database of thousands of newspapers dating back to the 1800s. In the case of over 150 of the papers, there are also digitized, searchable pages. And it's all available via its new Chronicling America API (technical details at our Chronicling America API profile).
It's always exciting to see those outside the web industry using an API to improve their sites. NY Times First Look blog has a great example of what Dallas Public Library has done with the Best Sellers API.