As the official handbook of the Federal Government, The United States Government Manual provides information on the agencies of the legislative, judicial, and executive branches. It also includes information on quasi-official agencies; international organizations in which the United States participates; and boards, commissions, and committees. The Manual begins with reprints of the Declaration of Independence and the U.S. Constitution.
There are hundreds of different departments and agencies such as the Bureau of Labor Statistics and the National Park Service. If you are searching a particular topic you can search using Google and type in the keywords related to your research topic followed by site:gov. This will limit your search results to government websites, and often the agency that is responsible for researching and gathering statistics on that topic will also appear in the top results.
For example, if I wanted information on the Farm Bill, I might construct a search in this way:
The results of this search show that I can look for information from the United States Department of Agriculture's website.
The Congressional Research Service is a "think tank" that provides reports to members of Congress on a variety of topics relevant to current political events. These reports, intended to provide congressmen with an overview of a particular issue, are often brief and may lists of pros and cons. The reports are not comprehensive as not all are available to the public.
The U.S. Government Accountability Office (GAO) is an independent, nonpartisan agency that works for Congress. Often called the "congressional watchdog," GAO investigates how the federal government spends taxpayer dollars.
auditing agency operations to determine whether federal funds are being spent efficiently and effectively;
investigating allegations of illegal and improper activities;
reporting on how well government programs and policies are meeting their objectives;
performing policy analyses and outlining options for congressional consideration; and
issuing legal decisions and opinions, such as bid protest rulings and reports on agency rules.
The CGP is the finding tool for federal publications that includes descriptive information for historical and current publications as well as direct links to the full document, when available. Users can search by authoring agency, title, subject, and general keywords, or click on "Advanced Search" for more options.