EDStats - The World Bank Education Statistics Database
EDStats is a World Bank site with much data on education, both via tables and databases. The Data Query System may be of particular interest for time-series data.
PovertyNet: Data on Poverty
PovertyNet has brought together a variety of datasets pertaining to the study of poverty, income inequality, and income distribution, along with other resources relevant for international development and economics. The many potentially-useful resources listed within this site include the World Income Inequality Database and PovcalNet. Be warned - the World Bank tends to change addresses for websites without warning.
World Health Organization Global Health Observatory (GHO)
The Global Health Observatory is a great source of data on numerous international/cross-national health-related topics. The site also contains many links to other sources of cross-national health data, such as the GHO Data Repository.
World Population Prospects
This database is produced by the United Nations' Population Division and contains cross-national data on basic population demographics and vital statistics (e.g. birth and death rates, maternal mortality, population by age and gender and urban/rural areas, median age of population). Data are available in five-year increments back to 1950 and with projections up to 2050.