Sources:

American FactFinder
Easy search tool for finding recent data published by the US Census.

Biographical Directory of the US Congress
Biographical information on every person to serve in either house of the US Congress since 1774.

Census State and County Quickfacts
Census statistics for all states and counties, and for cities and towns with more than 5,000 people.

Congressional Directory
Contains contact information for all members of Congress as well as lists of other officials of the Federal Government.

Consumer.gov and Consumidor.gov
Consumer information in both English and Spanish.  Includes information dealing with credit, debt, identity theft, and avoiding scams, as well as budgeting, opening a bank account, shopping for prepaid cards, and managing money in general.

Consumer Expenditure Statistics
Information regarding the buying habits of American consumers.

Directory of State Tax Forms
Tax forms for all 50 US States.

FEDSTATS
Collection of data and statistics related to all areas if the US Federal Government.

GPO Access
Provides free access to databases and other online tools produced by the federal government.

NANPA (North American Numbering Plan) Area Code Lookup
Area code information for the United States.

National Agriculture Library
Provides information on books and articles related to agricultural topics in the United States.

Patent Database
Full-text database containing information on patents issued in the United States.

THOMAS (Library of Congress)
Library of Congress database of congressional information including all bills considered in both houses.

US Copyright Office
Information about US Copyrights including how to copyright your material as well as help finding existing copyrights.

US Government Manual
Information regarding the operation of the US government.