If you're just beginning your search, these resources might be a great place to start:
Data.gov is the home of the U.S. Government's open data. Here you will find data, tools, and resources to conduct research, develop web and mobile applications, design data visualizations, and more.
govinfo is a service of the United States Government Publishing Office (GPO), which is a Federal agency in the legislative branch. govinfo provides free public access to official publications from all three branches of the Federal Government. govinfo replaced the FDsys website in December 2018. GPO launched govinfo in February 2016 as a beta website to replace its predecessor, GPO’s Federal Digital System (FDsys). Learn about the transition.
In addition to providing an advanced, metadata-powered search experience, govinfo also includes a content management system and a standards-compliant preservation repository.
These three components comprise GPO’s world-class system for the comprehensive management of electronic information:
GPO is seeking to become the first Federal agency to be named as a Trustworthy Digital Repository for Government information through certification under ISO 16363.
This is the official online source for IRS tax forms, instructions, and publications.
"Science.gov searches over 60 databases and over 2200 selected websites from 15 federal agencies, offering 200 million pages of authoritative U.S. government science information including research and development results. Science.gov is governed by the interagency Science.gov Alliance."
USA.gov is the United States government official web portal. This resource allows users to search government information both by topic and agency. There are also links where you can contact the Federal, State, Local, and Tribal governments, as well as elected officials. The topical list of government information is a great place to start for general searches.
Worldometers offers real time statistics on world population, government and economics, social media, and the environment, to name a few. This is a great resource for up-to-date statistical information around the world.
Websites for historical documents of the United States government:
America's Historical Documents from the National Archives
U.S. History and Historical Documents from usa.gov
Historical Documents from the Office of the Historian of the United States