Miriam Robbins Midkiff has written a great article on her AnceStories blog listing all the known state census records that are available online. She writes:
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!
Miriam's article continues with even more explanation and then lists online state census records available for 24 states.
This article is a keeper! I copied-and-pasted it to my list of "things I may need to know someday." You might want to do the same.
You can find Miriam Robbins Midkiff's list of State Census Records Online at http://ancestories1.blogspot.com/2009/12/state-census-records-online.html.
While on that site, you might want to check out Miriam's other articles on the AnceStories blog. Good stuff!