U.S. Federal and State (see 1890 Census on previous page) censuses through 1940 and an index of the 1890 NYC Police Census may be searched and viewed for free at www.familysearch.org and at the fee-based site www.ancestry.com, which offers more advanced search features. The Center for Jewish History subscribes to Ancestry Library Edition, which is provided free of charge to patrons on-site at the Center for Jewish History. To determine which State censuses are available at FamilySearch, go to www.familysearch.org/search and enter in Collection Title search box “[Name of State] census.” At Ancestry, go to Search/Card Catalog and enter in Keyword(s) search box “ [Name of State] census.”
This website provides census questions and enumerator instructions for all U.S. federal censuses from 1850 on.
The National Archives has created an extensive online research guide on U.S. census records, including a list of valuable genealogical clues to be found in each federal census.
1890 U.S. Census form, source: http://www.census.gov/pubinfo/www/broadcast/photos/historical_census/004299.html, via Wikimedia Commons