HeritageQuest® Online is a comprehensive treasury of American genealogical sources—rich in unique primary sources, local and family histories, and finding aids. The database provides genealogical and historical sources for more than 60 countries, with coverage dating back as early as the 1700s.
Check out this video on how to use HeritageQuest (26min 48sec). It includes a tutorial on how to search census records, as well as a demonstration of the search options. HeritageQuest is free; in order to save information and build a customized family tree, an Ancestry.com subscription might be required.