While NARA hosts billion of actual records of different nature covering a broad range of topics, its website offers plenty of integrated services and is regularly updated and improved.
The US National Archives and Records Administration is an independent Federal agency devoted to collect and manage official records
Being an integrated service, almost all NARA's branches provide location access with content description. For some historical references, full-text documents are available in different format
NARA is still a traditional archive: 10 billion paper records, 30 million photographs and 3 million maps and charts are stored in more than 30 buildings around the country. Their digitalization process is still unfolding
Though not PS specific site, NARA is a huge archive gathering selected documents from the executive, legislative and judicial branches of the US Federal Government
In most sections/branches records are selected and added weekly, especially the online news and events sections