The US government is one of the world’s largest publishers, both in paper and on the web.  More than 200 years ago Congress authorized the sending of free copies of certain government publications to libraries around the country.  This developed into the Federal Depository Library Program (FDLP), with more than 1,100 public, academic, and government libraries as members.  Each of these libraries receives government materials for free, and is required to make them available to the public.

The Federal Depository Library Program is run by the Government Printing Office (GPO), which recently changed it’s name to Government Publishing Office in order to indicate it is now more involved with electronic data rather than paper.

If you are looking for government publications and other federal information, give these online sources a try: 

Federal Digital System (FDsys)
Offers free access to authenticated government publications.  Most of the materials are arranged in 50 large collections within the database.
http://www.gpo.gov/fdsys/
(Also the GPO has a new website in beta called govinfo)

Catalog of US Government Publications (CGP)
A digital descendent of the Monthly Catalog of Government Publications.  The CGP covers most federal documents from 1976 forward, and is slowly gathering in thousands of earlier documents.  Be aware in most cases the publications in CGP are not full-text.  Also CGP will give you the Sudoc number for paper publications, and the URL for web-based ones.
http://catalog.gpo.gov/

USA.gov
USA.gov is the United States government search engine.  Consider it the Google search for .gov, .mil, and parts of the .edu domains.  If you are looking for information that you expect will be on a government website (federal, state, or local), this is a good place to search.
http://www.usa.gov

MetaLib
MataLib is a federal search engine.  This search engine simultaneously searches across 10 government databases.  This tends to make the searches slower but may save time if you were planning to search extensively.
http://metalib.gpo.gov

Homeland Security Digital Library
A database of publications related to homeland security. Most are government documents covering topics such as terrorism, border issues, climate change, immigration, pandemics, school violence, and food security.  This database contains more than 150,000 full-text documents.
https://www.hsdl.org/

Science.gov
Like MetaLib, Science.gov is a federated search engine.  It searches for scientific research materials in more than 60 databases and 2000 websites produced by 15 federal agencies.  Searching can be slow due to the large volume of data sources.
http://www.science.gov/

Our Catalogue
Searches hundreds of thousands of Canadian federal publications.  Note that not all publications are available full-text on the web.
http://publications.gc.ca/site/eng/ourCatalogue.html

CyberCemetery
When a government agency goes out of business, what happens to its website and publications?  It can go here!
http://govinfo.library.unt.edu/

American Memory
The Library of Congress’ website for American History.  A location of oral histories, old photographs, sheet music, and other old stuff!
http://memory.loc.gov/

American FactFinder
American FactFinder is the Census Bureau’s storehouse of information from recent censuses and surveys.  The data can be overwhelming!  A good place to start is Community Facts, which lets you easily find data about a county, town, or zip code.
http://factfinder.census.gov/

CIA World Factbook
The World Factbook is an authorative guide to the countries of the world and even for some places that are not actually countries (like Greenland, Falkland Islands, Svalbard, etc.).  Each entry contains notes on history, geography, politics, people and society, economics, and more.
https://www.cia.gov/library/publications/the-world-factbook/

WhiteHouse.gov
The president’s official website gives you his daily schedule, texts of speeches and presidential actions, and the administration’s viewpoints on issues of the day.
https://www.whitehouse.gov/