For more see the Government Information portal at Columbia. If you are unfamiliar with how the American Government is set up (it's confusing!) please consult this fabulous set of resources fromUSA.gov.
National Archives: Links and indexes to government publications in the National Archives.
Science.gov : Provide users with access to more than 200 million pages of authoritative federal science information including research and development results.
USA.gov: Official guide to government information and services