There are numerous on-line sources of data and other information about the U.S. population, as well as the global population. This page focuses on the U.S. population, but you can use some of the links to broaden your search.
The U.S. Census Bureau published a nice page, with population estimates for the nation and for state. There are links with other data sources and further information about the Census. The Center for Disease Control publishes weekly information on morbidity and mortality. The National Center for Health Statistics publishes vital records data--births, marriages, divorces and deaths--as well as . Two of the institutes at the National Institutes of Health (NIH) play major roles in studying the American population. The National Institute of Child Health and Development (NICHD) funds much of the research on childbearing and marriage that is being conducted. The National Institute on Aging (NIA) focused (obviously) on population aging and related policy issues.
The Population Reference Bureau (PRB) is a private organization that disseminated a great deal of information on population patterns. Their page includes an excellent links page of on-line resources and the 1997 World Population Data Sheet. The Demographics and Population Section of the Virtual Library has global and American resources.
Dow Jones has a journal with the catchy name, American Demographics. The page has some information on the journal, which is in Perkins and the Markets & Management reading room.