U.S. Government Manual - offical directory of departments, agencies (and quasi-official agencies), international organizations in which the US participates, boards, commissions, and committees

Searchable sites:

U.S. Federal Agencies Database

U.S. Federal Government Agencies Directory

Lists:

Federal Web Locator

Who's Who in the Federal Government

Official Federal Government Web Sites - Executive Branch

Federal Agencies & Commissions

Other Resources:

Federal Advsisory Committee Database - an excellent resource for charters, membership, and reports covered under the Federal Advisory Committee Act of 1972

Plumbook - top Civil Service jobs and their salaries