African Census Analysis Project is probably the best site to look for African census data. Contains data from many countries from different years. Sign up is required.
Center for Demography and Ecology is a research cooperative for social scientific demographic research.
Demographic and Health Surveys collects data in developing countries about population, health, and nutrition programs.
Family Life Surveys contain data on Malaysia (1976-77, 1988-89), Indonesia (1993, 1997, 2000), Guatemala (1995), and Bangladesh (1996).
Homeland Security offers data on immigration into the U.S.
Migration Information Source provides statistics on international migration and refugee trends.
Population Reference Bureau has population information on important social, economic, and political issues.
Table Maker, by the Alan Guttmacher Institute, has data on sexual and reproductive health, education, economics, etc.
World Health Organization Statistical Information System (WHOSIS) has links to worldwide data on health issues.U.N. Economic and Social Development: Population Programmes has links to many U.N. programs that deal with population issues.
U.S. Census Bureau is always a great source for population studies.