Comprehensive summary of statistics on the social, political, and economic conditions of the United States, from 2013 to the present. Volumes covering 1879-2012 are on the US Census site.
Online edition of a three volume set, covering the Americas, Europe, and Africa, Asia, and Oceania.
Useful to genealogy research are the population schedules of the census, which lists individual heads of households. They can be found at the UCLA library on microfiche and some digitized copies are available in the Ancestry Library database. The nonpopulation census schedules would be another good source.
An extensive collection of family history research databases, including the US federal census images from 1790 to 1950. Please note there are differences in features and content between Ancestry Library Edition and Ancestry.com subscriptions for individuals.