The National Archives has a great deal of valuable information for the genealogist and family historian. This is probably the most well-known agency to genealogists. NARA is the repository of our government's records, many of which include information about our ancestors. There is a whole section for genealogists and family historians.
NARA's magazine Prologue has many useful articles about their collections, and each issue has an article for genealogists. Selected ones are online back to 1973, and an index, by year, back to 1982.
Other useful online publications include Citing Records in the National Archives of the United States and other genealogy guides to using their collections that researchers can read before they visit. You can also buy their publications relevant to your research such as U.S. Armed Forces: A Guide to the Records of Military Agencies in the National Archives Relating to American Women.
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Be sure to check out the Archival Databases (AAD), which provides "online access to records in a small selection of historic databases preserved permanently in NARA," and the Archival Research Catalog along with its guide for genealogists, which is the online catalog of NARA's nationwide holdings in the Washington, DC, area, Regional Archives, and Presidential Libraries. Also check our their new Online Public Access system, which aims to be "a centralized, streamlined means of searching multiple National Archives resources at once."