Thanks to a link I found in the Genealogical Research Resources Group started by Denise Olson, I found myself at the Historic Missouri Newspaper Research Project. The project is a collaboration among several institutions in Missouri, including Lincoln University, the University of Missouri- Columbia Libraries, the Kansas City Public Library, and the State Historical Society of Missouri.
There are not a lot of newspapers in the project right now, but those that are there are rich in historical data. Additionally, I think the software base of the project, called Active Paper, allows easier and more efficient searching and retrieval than some commercial newspaper archives. Active Paper is a product of Olive Software, a seven year old Silicon Valley-based company.
I've found a lot of stories about my northwest Missouri ancestors in the papers in this project. The search function is so easy to use that I find myself spending hours playing with it!