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- Historic St. Mary’s City
- St. Mary’s County Newspaper Archive from the St. Mary’s County Library
- St. Mary’s County Historical Society, including the Old Jail Museum
- St. Mary’s County Genealogical Society
- The John Hanson Briscoe Historical Project. Free online book about the former Speaker of the Maryland House of Delegates, with chapters covering other topics in St. Mary’s County History. Oral History transcripts related to this project are in the SlackWater Oral History Collection. (link to the 42 interviews)
- Patuxent River Naval Air Museum
- Calvert Marine Museum
- St. Mary’s County Historic Preservation Commission
- Southern Maryland Studies Center, College of Southern Maryland