American Fact Finder U.S. Census Bureau.
FedStats Easy access to statistics produced by more than 70 agencies of the United States Federal Government.
Population Reference Bureau’s Country Statistics & Reports
Statistical Abstract of the United States Collection of statistics on the social and economic conditions in the U.S.
United Nations Population Information Network (POPIN)
United States Census Bureau

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Yahoo!: Census Bureau
Librarians Index to the Internet: Country Studies
BUBL: Population
DMOZ: Census