The list below provides a variety of population-related links. Some of the specialist links use the word DEMOGRAPHY when talking about population.
One of the best sites to start is the excellent U.S. Census site. This not only has excellent free information from the USA census but also very detailed data on every other country. This is held in the IDB section [International Data Base]. There are also lots of useful population pyramids.
U.S. Census sites:
US Censusthe best of the population sites home page.
The IDB Pagefull of very useful population information on every country you can think of.