Last year, an article was written by Diana Crisman Smith for the Association of Professional Genealogists Quarterly. She warned of the possible shutdown of the U. S. Government, and how it would impact persons using Federal records for their historical research.
Luckily, we don't seem to have a shutdown looming at the moment. But the article was a good review of exactly what resources are available to the community of researchers in our nation. Here's a few of the repositories mentioned by Diana, what they can provide in the way of documents and records:
National Archives and Records Administration (NARA)
census records, immigration records, military records, homestead records. see www.archives.gov
Library of Congress (LOC)
books, microfilm, newspapers, periodicals, directories, manuscripts and maps. see www.loc.gov
Department of the Interior
park services, civil war soldiers database, civil war battles and battlefields and all military units involved. see www.doi.gov
The Bureau of Land Management holds records of land owned by the federal government, excluding the thirteen original colonies, Hawaii, Kentucky, Maine, Tennessee, Texas, West Virginia and the District of Columbia. The records of the first sale of public land to individuals. see www.blm.gov
The Copyright Office
registration and verification for copyrights for works of art or literature. see www.copyright.gov
Diana Crisman Smith has some suggestions to those researchers using these and other government resources. She highly suggests to transcribe all the information that you need the first time that you find it, including full citation. Take a screen shot of a web page that has helpful information on it. Things change quickly on the web, and that web page may not be available again. She adds that assuming resources will always be available is a mistake, so use these resources as soon as you need them.
In the event of another government shutdown, various genealogy websites may have information from the U.S. Federal Government websites mentioned here. But, let's hope we don't have to worry about that.