The Brazilian government's geographic department (Instituto Brasileiro de Geografia e Estatística-roughly equivalent to the U.S. Census Bureau) has compiled an fantastic interactive world factbook (available in English and Spanish as well as Portuguese). The ease of navigation allows the user to conduct a specific search of simply explore demographic, economic, environmental and development data on any country in the world.
Real time statistics for current population of any country. Real time data on population, births, deaths, net migration and population growth.
This site shows various demographic statistics for every country including some based on projections in demographic trends in the given country. If the current trends hold (which they won't, but that is still an interesting measure), the entire Japanese population will disappear in 1,000 years according to this Global Post article.