World Population Prospects
The United Nations Population Division has released its 2010 revision of the World Population Prospects providing population estimates and projections for all countries and regions of the world from 1950 to 2100
The UN data, updated every two years, cover population size, fertility, mortality, migration and population ageing. They can be accessed in a variety of forms:

Under the Data heading, users can access an on-line database of population and population density, detailed indicators, country profiles and Excel files covering the main topics.
The Key indicators section provides access to data in the form of self-sorting tables and graphs. In the Figures section, users can click on Population by Age and Sex to obtain population pyramids for different periods.
The Documentation and Methodology sections include a glossary and definitions, along with the assumptions, methods and models used to make this revision.

There is also a link to World Urbanization Prospects, a website that presents the estimates and projections of the urban and rural populations of all countries in the world and of their major urban agglomerations.