With the announcement Wednesday from Ancestry that they had added the Nebraska State Census of 1885 to their collection, I realized that there is a plethora of state census records now available online, something that wasn't in existence ten, even five, years ago! FamilySearch has added many of these records to their pilot program as well.
How are state censuses helpful in genealogy? They were often held in the years between the decennial federal censuses; many were enumerated in years ending in "5," such as 1885, 1895, etc. They are also a great substitute for the missing 1890 Federal Censuses. If your ancestor lived in a territory prior to statehood, chances are you'll find him or her enumerated several times, since territories often took multiple censuses in order to prove they had enough residents to be admitted to the Union. Finally, state censuses contain answers to questions that aren't normally asked in federal censuses: complete birth dates, specific birth locations, detailed information about your ancestor's farm (acreage owned, number of cattle, pounds of butter yielded in the past year, or bushels of wheat harvested). They often show lists of marriages by township in states that did not yet require keeping of vital records; names of churches and industries within the township, showing a larger picture of the community; and even details such as whether your ancestor served in the military or had a log or frame home!
I decided to compile a list of state census records available at the two websites. Surprisingly enough, there is very little duplication between them, which increases the amount of records available. FamilySearch records are available for free, but are offered in various forms: record images, indexed and searchable; record images, unindexed and therefore only browsable (lots of manual searching involved, especially if you're not sure in which county your ancestor lived); and no images, with the records' transcribed indexes available for searching. Ancestry records require a subscription, but many public libraries offer the Library Edition for free to its patrons. Ancestry often combines territorial censuses with state ones. On both websites, not every county is represented for each state census available for a multitude of reasons: records were lost or destroyed; the counties in question were not yet formed; or the website has not uploaded all available records for that census year. Also, not every state census year is available online yet. Michigan, for example, had a state census in 1884, but Ancestry only has the one for 1894.
What you won't see here are colonial censuses, tax lists, or voter registrations, merely because that's a slippery slope into census substitutes. I have included only state or territorial censuses conducted by each state's (or territory's) official government, or--in the case of a few states in 1885-- the federal government. A few states, such as Iowa and Washington, also conducted some county censuses and I have mentioned these as well. The latter are also available in transcribed format only at the Washington State Digital Archives website for free.
Alabama - Ancestry - 1820, 1850, 1855, 1866; FamilySearch - 1855 & 1866, indexes only
Arizona - Ancestry - 1864 (territorial)
California - Ancestry - 1852 (territorial); FamilySearch - 1852 (territorial), index only
Colorado - Ancestry - 1885; FamilySearch - 1885
Florida - City of Hollywood - 1935 for City of Hollywood only; FamilySearch - 1885, 1935, 1945; and Ancestry - 1867, 1875, 1885, 1935, 1945
Illinois - Ancestry - 1825, 1830, 1835, 1845, 1855, 1865; FamilySearch - 1855 (index only), 1865
Iowa - Davenport Public Library - 1856, 1885, 1925 for Scott County only; Ancestry - 1856, 1885, 1895, 1905, 1915, 1925 (database also contains some head of household only censuses and other special censuses from 1836-1897)
Kansas - Ancestry - 1865, 1875, 1885, 1895, 1905, 1915, 1925 (database also contains voter lists for 1855 - 1859)
Massachusetts - FamilySearch - 1855, 1865
Michigan - Muskegon County Genealogical Society - 1884, 1894 for Muskegon County only; Ancestry - 1894
Minnesota - Minnesota Historical Society - 1865, 1875, 1885, 1895, 1905 indexes; FamilySearch - 1875, 1885, 1895; and Ancestry - 1865, 1875, 1885, 1905 (database includes four territorial censuses prior to 1858 statehood)
Mississippi - Ancestry - 1817-8, 1818, 1820, 1822, 1822-23, 1823, 1824, 1825, 1830, 1837, 1840, 1841, 1845, 1850, 1853, 1860, 1866 (database includes six territorial censuses prior to 1817 statehood)
Missouri - St. Charles Historical Society - 1876 (index for St. Charles); Ancestry - 1844, 1856, 1857-8, 1868-9, 1873, 1880, 1881
Nebraska - Ancestry - 1870, 1874, 1875, 1876, 1877, 1878, 1879, 1880, 1881, 1882, 1883, 1884, 1885 (database includes two territorial censuses prior to 1867 statehood)
Nevada - Ancestry - 1875
New Jersey - New Jersey State Archives (Passaic County and Atlantic City only) - 1885; Ancestry - 1895
New York - Western New York Genealogical Society - 1865 (partial); FamilySearch - 1865, 1892, 1905; and Ancestry - 1790, 1880, 1892, 1905, 1915
North Carolina - Ancestry - 1784-7
North Dakota - Ancestry - 1915, 1925 (also Dakota Territory - 1885)
Oklahoma - Ancestry - 1890, 1907 (both territorial censuses)
Oregon - Ancestry - 1850 (territorial)
Rhode Island - FamilySearch - 1915, 1925
South Dakota - FamilySearch - 1905, 1915, 1925, 1935; and Ancestry - 1895 (also Dakota Territory - 1885)
Washington - Ancestry - 1891, 1892, 1898; and Washington State Digital Archives - 1891, 1892, 1898 (both databases include 15 county or territorial censuses prior to 1889 statehood)
Wisconsin - FamilySearch - 1855, 1875, 1885, 1895, 1905; and Ancestry - 1895, 1905
Several printed sources I highly recommend to increase your knowledge about these resources:
State Census Records by Ann S. Lainhart - a nice overview of each state that held a census, what is available, and where (usually offline repositories)
Censuses Substitutes and State Census Records by William B. Dollarhide - Volume I (Eastern States) and Volume II (Western States) - these volumes go into much greater detail than Ann Lainhart's book and list Family History Library microfilm numbers of many of the state census records and substitutes (tax records, voters lists, etc.).
New York State Censuses and Substitutes by William B. Dollarhide - if you have New York State ancestors, you need this book! New York state census records are some of the most difficult to find, as they are not stored in one central repository, but in many town halls and court clerks' offices around the state. Even if you have Dollarhide's Censuses Substitutes and State Census Records Volume I (Eastern States), you will need this book because it is much more comprehensive than the latter, which only shows the state census information from one New York county.