The main search engines for U.S. Government information are:
FDSys (Federal Digital System from the U.S. Government Printing Office)
FDSys provides access to official U.S. government publications from all three branches of government. It's more complicated to search than the Catalog of Government Publications, so the following tutorials may be helpful:
Basic Searching in FDSys
FDSys Educational Webinar Recordings. Most of these presentations are 1-1.5 hours long.
-Introduction to GPO's Federal Digital System
-Tracking Federal Regulation in FDSys
-Navigating Government Information Using GPO's Federal Digital System
-Advanced Navigation in GPO's Federal Digital System
-Tracking Legislating in GPO's FDSys
Catalog of Government Publications (CGP, U.S. Government Printing Office)
"The CGP is the finding tool for federal publications that includes descriptive records for historical and current publications and provides direct links to those that are available online. Users can search by authoring agency, title, subject, and general key word, or click on 'Advanced Search' for more options." The Catalog of Government Publications has been superseded by FDSys, but it's still useful for searching for older publications.
USA.gov: The U.S. Government's Official Web Portal
This website includes a detailed list of Federal, State, Local, and Tribal agencies, listed under the "Find Government Agencies" tab.
Google has quietly discontinued its U.S. Government (aka Google Uncle Sam) web search. There is no link on the Advanced Search page, and government information librarians noticed that it disappeared in mid-June 2011.
You can still use Google to find government information, by adding "site:gov" to your search. Or you could go to the Advanced Search and enter .gov in the "Search within a site or domain" box. Unfortunately, this will not replicate the old Google Uncle Sam search,which included state, county and military websites.
To search for California state documents, add "site:ca.gov" to the end of the search.
For San Francisco City & County documents, add "site:sfgov.org" to the end of the search.