United Nations Population Information Network
World population trends, prospects, current population with links to other sites.

United Nations Statistics Division.  
Global statistics with links to many related agencies.

Demography.
Web page from Trinity University, with links to census and international population data, immigration, cities and urbanization and maps.

World Gazetteer. 
Current population for U.S. cities, towns and countries.

Cities of the World. 

City populations with country maps.

State of World Population Report. 
Published by the United Nations Population Fund, it provides a lengthy report on trends in world population.