http://www.LIB.LSU.EDU/GOV/FEDGOV
Comprehensive, well-organized site compiled by Louisiana State University Libraries. Federal agencies are listed alphabetically, by branch, or by type.

http://www.FEDSTATS.gov
Fed Stats provides easy access to statistics and information produced by more than 100 Federal Government agencies.

http://www.whitehouse.gov/government/independent-agencies.html
An alphabetical listing of Federal Agencies and Commissions.

http://www.FIRSTGOV.gov
Official web portal of the US government contains comprehensive information on government resources, services and forms for citizens, businesses and much more.