Today's resource is the wonderful list compiled by the Research Society of American Periodicals (RSAP). "The RSAP is an interdisciplinary organization of scholars interested in American magazines and newspapers. It publishes the journal American Periodicals, sponsors panels at the annual meeting of the American Literature Association, and has a free moderated discussion list."
The Research Resource Page on RSAP's site links to a wealth of digitized 19th, 20th and 21st century American periodicals, zines and other alternative literature. Larger digitization projects like Cornell and Michigan's Making of America collections are featured, as well as smaller local, historical, ephemera and genre collections.
Brooklyn Public Library has digitized nineteenth-century editions of the Brooklyn Daily Eagle.
The Library of Congress recently added a beta search to the Chronicling America collection, which shows highlighted search terms in digitized thumbnails. More newspapers are regularly added to that collection as well. UPDATE: The Library of Congress blog just announced more changes to the Chronicling America site today. Notable improvements are new date range search capabilities and persistent URLs for bookmarking.
For anyone looking for historic newspapers for primary sources, the RSAP's resource list is an excellent place to start.