The National Contact Center (NCC) of USA Services responds to thousands of requests every business day about federal programs, benefits, and services. They either provide information directly or locate the source of assistance. Their website provides the toll-free number and email address of the NCC. For finding an answer on your own, you could use their knowledgebase.
The Government Departments and Agencies page of FirstGov contains links to information about federal government branches, local, state, territorial, tribal, and worldwide governments, an A-Z index of federal departments and agencies, and more.
Firstgov's Contact Your Government page contains a link to U.S. Government Telephone and e-mail Directories. The same directories are included in the following list.
The preface of the U.S. Government Manual states "As the official handbook of the Federal Government, The United States Government Manual provides comprehensive information on the agencies of the legislative, judicial, and executive branches. The Manual also includes information on quasi-official agencies; international organizations in which the United States participates; and boards, commissions, and committees....The Manual is published as a special edition of the Federal Register...Its focus is on programs and activities. Persons interested in detailed organizational structure, the regulatory documents of an agency, or Presidential documents should refer to the Federal Register or one of its other special editions...."
"The content on ExpectMore.gov is developed by the U.S. Office of Management and Budget and Federal agencies. Together, we assess the performance of every Federal program and hold ourselves accountable for improvement."