This list from the General Services Administration is the US government’s official list of .gov domains (there are undoubtedly others that aren’t publicly admitted, as well as many public .gov sites that are missing). Many of the domains registered to intelligence agencies are extremely obscure. Below, I’m listing all of these domains as clickable links. Not all of them are working websites.
Registered to the CIA:
df.gov
ic.gov
istac.gov
odci.gov
opensource.gov
osde.gov
osdls.gov
ttic.gov
ucia.gov
Registered to the NSA:
Registered to the Defense Department (NSA, NRO, DIA, etc.):
nrojr.gov
siteidiq.gov
Registered to the Office of the Director of National Intelligence:
ce-ncsc.gov
intelink.gov
ncix.gov
osis.gov
pix.gov
qart.gov
ugov.gov