This being 2010, we are now all part of the newest decennial census, started in 1790, and conducted every 10 years by the US government.
The questions asked have varied greatly over the years, at some point providing detailed answers to genealogical questions such as family relationships, ages, dates of birth, how long a citizen, year of naturalization, number of children born vs. number of children living, whether or not a first marriage, etc.
At other times, census questions do not yield particularly helpful information for genealogical researchers. This is unfortunately true of the 2010 census, which asks lots of interesting racial questions, but leaves out the useful information from previous censuses.
I find it curious that the 2010 census asks for very detailed information on a great many possibilities, but groups all whites as the same. For example, there is a misconception that because a person is white, he or she is a WASP, meaning white anglo-saxon protestant. However, many white people are Celts or Vikings and Catholic or Quaker, etc. We are diverse, too.
State censuses are also useful sources of information.
Don't forget to look for recreated census records for burned counties; these are compiled from various records such as taxes, voting lists and the like.
Here are a few useful links:
http://www.census.gov/
http://www.archives.gov/genealogy/census/index.html
http://www.archives.gov/publications/record/1998/01/family-history.html
http://search.ancestry.com/search/default.aspx?cat=35
http://www.footnote.com/documents/60505077/census_us_federal_1930/
http://www.heritagequestonline.com/hqoweb/library/do/index
