United States Census Indexes
The information in an index may be incomplete or incorrect. If you have reason to believe your ancestor should have been in the census, search the census regardless of the information in the index. In large cities, learn the person's address by searching the city directory. for the same year as the census (see the “Directories” section of this outline). Then look for that address on the original census schedules starting in 1880. Prior to 1880 it may be helpful to learn the ward where a person resided.
Record Search has indexes for the 1850-1900 and 1920 censuses. Images are available for most of them.
Ancestry has online images & every name indexes for 1790-1930, 1850 & 1860 slave schedules, 1890 Veteran's Schedule, and 1930 Merchant Seamen. Ancestry.com includes the census index information that is found in Accelerated Indexing Systems U.S. Census Indexes (on Microfiche).
Heritage Quest has online indexes for 1790-1820, 1860-1920 & 1930 (Connecticut, Delaware, Maryland, Texas, and Virginia) and images for 1790-1930.