Miriam Midkiff has done a service for us all. In her Blog, AnceStories: The Stories of MY Ancestors, she has enumerated all of the state censuses made available to us through Ancestry.com and FamilySearch.org.
This is helpful because there are some differences in the coverage. It is also helpful because the states censuses covered by FamilySearch are more or less buried in their Pilot Search Project. Unless you remember to frequently check on what that project has added, you could easily miss valuable new entries.
You can check out Miriam's listing at: http://ancestories1.blogspot.com/2009/12/state-census-records-online.html