This database has been up for a few years, but I think that it deserves another mention. http://fisher.lib.virginia.edu/collections/stats/histcensus/
The University of Virginia Library has compiled the population and other statistics for all of the federal censuses from 1790-1960. You can select which statistics to view and compare, and view all 50 states or by county within states. These databases are statistical only, and contain no information on specific households.
But this statistical information is nearly as important as the actual census returns themselves, for it provides context. As much time as we spend investigating our ancestors, we must also remember that they did not live in a Family Group Record world. They were members of a community, and the demographic information contained in this database will tell you about that community.
Some of the information is particularly useful for genealogists, if you compare your family to the statistics for the county in which they lived. Information on the population by size of household, gender, age, and race, information on the ownership and size of farms, occupations, religion, etc., are all available for searching.