The United States Government's Official Web Portal (formerly firstgov.gov)

The Federal Citizen Information Center in Pueblo, Colorado is a very good free information resource.  The Center staffs the National Contact Center daily and provides a vast array of information and assistance.
The National Contact Center, (or 1-800-FED-INFO)

Alphabetical listings of federal agencies and commissions

• http://www.whitehouse.gov/government/independent-agencies.html
• http://www.lib.lsu.edu/govdocs/federal/list.html

Federal Intelligence Community Agencies (currently 18)

• Director of National Intelligence
• Undersecretary of Defense for Intelligence
• Air Force Intelligence
• Army Intelligence
• Central Intelligence Agency
• Coast Guard Intelligence
• Defense Intelligence Agency
• Department of Energy
• Department of Homeland Security
• Department of State
• Department of the Treasury
• Drug Enforcement Administration
• Federal Bureau of Investigation
• Marine Corps Intelligence
• National Geospatial-Intelligence Agency
• National Reconnaissance Office
• National Security Agency
• Navy Intelligence

Federal Law Enforcement Agencies listed

Top 100 federal contractors of 2007 (Washington Technology magazine)

List of federal prisons

Federal Acquisition Regulation (FAR)