The National Archives & Records Administration (NARA, or Archives.gov) is the major archives website for government information—interpreted broadly so as to include material such as military and older Census records.
Here are the major sections of the Archives.gov website of interested to researchers:
And here are some specific sites of interest for browsing and just for fun:
NOTE that the Library ceased receiving new tangible government documents in Spring, 2016, and withdrew large numbers of tangible documents during the previous year.
NOTE: some of the following reference guides were published some time ago. They are still valuable resources for older collections of archives but be aware that they will not refer to newer material or to newer methods of access.
NOTE: NARA publications dated prior to 1986 are classified GS 4.; after that date, AE 2.
The Library has other publications from the National Archives of interest to researchers, especially items in these series:
Both of these substantial volumes identify repositories of papers, oral history tapes and tape transcripts:
Guide to the Research Collections of Former U.S. Senators, 1789-1995. Y 1.1/2:14218.
Guide to the Research Collections of Former Members of the U.S. House of Representatives, 1789-1987. Y 1.1/2:13874.