Federal Politics:

First Gov - Link to the Government
White House
(includes Office of Homeland Security)
House of Representatives
Senate
Republican National Committee
Democratic National Committee

Key Links:

About the U.S. - good for research!
(US Information Agency)
Federal Courts
(including Supreme Court)
FedWorld
(government documents)
Vote Smart - good for research!
(details on politicians)
Cabinet & other officials
(biographies)

Major Agencies, etc.:

AMTRAK (National Railroad Passenger Corp.)
ATF (Bur. of Alcohol, Tobacco & Firearms)
BOP (Bureau of Prisons)
CBO (Congressional Budget Office)
CDC (Centers for Disease Control)
CIA (Central Intelligence Agency)
CPSC (Consumer Product Safety Comm.)
CSB (Chemical Safety Board)
DEA (Drug Enforcement Administration)
EEOC (Equal Employment Oppty. Comm.)
EPA (Environmental Protection Agency)
FAA (Federal Aviation Administration)
FBI (Federal Bureau of Investigation)
FCC (Federal Communications Commission)
FCA (Farm Credit Administration)
FDA (Food & Drug Administration)
FDIC (Federal Deposit Insurance Corp.)
FEC (Federal Election Commission)
FEMA (Fed. Emergency Management Agency)
FHWA (Federal Highway Administration)
FRB (Federal Reserve Board)
FTC (Federal Trade Commission)
GAO (General Accounting Office)
GPO (Government Printing Office)
GSA (General Services Administration)
HCFA (Health Care Financing Admin.)
INS (Immigration & Naturalization Service)
IRS (Internal Revenue Service)
ITA (International Trade Administration)
LOC (Library of Congress)
LSC (Legal Services Corporation)
 

Executive Office of the President:

CEA (Council of Economic Advisers)
CEQ (Council on Environmental Quality)
DPC (Domestic Policy Council)
NEC (National Economic Council)
NSC (National Security Council)
OA (Office of Administration)
Office of Faith-Based & Community Initiatives

Federal Departments:

Agriculture
Commerce
(Census, ITA, NOAA-weather, Technology, USPTO)
Defense (Army, Navy, Air Force, Marines, National G.)
Education
Energy
Health & Human Services
(CDC, FDA-food/drug, HCFA-medicare/medicaid, NIH)
Housing & Urban Development
Interior (parks, fish & wildlife)
Justice
(DEA-drugs, FBI, INS-immigration, BOP, US Marshals)
Labor
State (travel & consular info)
Transportation
(Coast Guard, FAA-air, FHWA, NHTSA-crash tests)
Treasury
(ATF, Customs, IRS-taxes, Mint, Secret Service)
Veterans Affairs

NA (National Academies - science, etc.)
NARA (Ntl. Archives & Records Admin.)
NASA (Ntl. Aeronautics & Space Admin.)
NCUA (Ntl. Credit Union Admin.)
NEA (National Endowment for the Arts)
NEH (Ntl. Endowment for the Humanities)
NHTSA (Ntl. Highway Traffic Safety Admin.)
NIFL (Ntl. Institute for Literacy)
NIH (National Institutes of Health)
NLRB (National Labor Relations Board)
NOAA (Ntl. Oceanic & Atmospheric Admin.)
NRC (Nuclear Regulatory Commission)
NSF (National Science Foundation)
NSA (National Security Agency)
NTSB (Ntl. Transportation Safety Board)
OPM (Office of Personnel Management - JOBS)
PC (Peace Corps)
PBGC (Pension Benefit Guaranty Corp.)
SEC (Securities & Exchange Comm.)
SBA (Small Business Administration)
SI (Smithsonian Institution)
SSA (Social Security Administration)
SSS (Selective Service System - "THE DRAFT")
USAID (US Agency for Intl. Development)
USGS (US Geological Survey)
USIA (US Information Agency)
USOGE (US Office of Government Ethics)
USPS (United States Postal Service)
USPTO (US Patent & Trademark Office)
VOA (Voice of America)
More...

OMB (Office of Management and Budget)
ONAP (Office of National Aids Policy)
ONDCP (Office of National Drug Control Policy)
OSTP (Office of Science & Technology Policy)
OWIO (Office for Women's Initiatives & Outreach)
PFIAB (Pres. Foreign Intelligence Advisory Board)
USTR (United States Trade Representative)