Newspapers.com World Collection is an extensive database that provides online access to 4,000+ historical newspapers. Dating from the early 1700s into the 2000s, Newspapers.com Library Edition contains full runs and portions of runs of well-known, regional, and state titles to small local newspapers in the United States and other countries.
Indigenous Newspapers in North America aims to present a diverse and robust collection of print journalism from Indigenous peoples of the US and Canada over more than 9,000 individual editions from 1828-2016. 
A discontinued project by Google to scan and index historic newspapers. The Google News Archive is still available and a great resource that makes many difficult to find newspapers available freely online. 
Historic U.S. Free Online Protestant Newspapers is a list of free online historic Protestant newspapers from across the Unites States. The list is broken down by state and dates range from the early 1800s to today. 
This online collection presents newspapers edited by Frederick Douglass (1818-1895), the African American abolitionist who escaped slavery and became one of the most famous orators, authors, and journalists of the 19th century. 
The Southern Baptist Historical Library and Archives is a worldwide center for the study of Baptist history. Governed by the Council of Seminary Presidents, the SBHLA is one of the major denominational collections in the nation and serves, by assignment of the Southern Baptist Convention, as the central depository and archives of SBC records.
The Restoration History Library includes digitized journals, books, and papers related to the Restoration Movement. 
The HathiTrust Digital Library is home to millions of digitized books and publications. HathiTrust was founded in 2008 as a not-for-profit collaborative of academic and research libraries now preserving 19+ million digitized items in the HathiTrust Digital Library. They offer reading access to the fullest extent allowable by U.S. and international copyright law, text and data mining tools for the entire corpus, and other emerging services based on the combined collection.
The HathiTrust Digital Library is home to millions of digitized books and publications. HathiTrust was founded in 2008 as a not-for-profit collaborative of academic and research libraries now preserving 19+ million digitized items in the HathiTrust Digital Library. They offer reading access to the fullest extent allowable by U.S. and international copyright law, text and data mining tools for the entire corpus, and other emerging services based on the combined collection.
Digitized by the Oklahoma Department of Libraries, this collection includes 285 prisoner newsletters, dating from 1937 - 1973, written by inmates at the Oklahoma State Penitentiary and Oklahoma State Reformatory. 