Census records are one of the most used tools for genealogy research. Most censuses can be found online. Most ancestry tree services, such as Ancestry.com, Familysearch.org, MyHeritage.com and many others will turn up census records in their searches. I find most of my own through Ancestry.com and Familysearch.org which I’ve linked to below. There is also a link to an AreaCodes map that you might find helpful.
National Archives Census Records – Resources for Genealogists
This is a good place to go to learn more about the census or are new to genealogy research.
(note: a current check of this site shows many dead links and the searches do not work. I am leaving it up, but this site appears to be outdated.)
Ancestry.com is typically a paid for service, however, you can sign up as a Free Registered Guest. As a registered guest, you can view their Free Index Collections, which include US Federal Census, and census for some foreign countries. [note, if you’ve had an ancestry account that you no longer pay for, you become a Free Registered Guest and can continue to use your account, you just get limited access].
FamilySearch.org is a free service. You need to sign up for an account, but there is no charge.
RESEARCH BY CATEGORY
RESEARCH BY STATE
RESEARCH BY COUNTRY