Footnote.com, a pay site, is offering a rather unique use of the 1930 census. Not only is it indexed so you can you search for your ancestors and see an image of the census page, but you can also contribute to the information available.
You can actually attach pictures, stories, and notes to the names in the census record. They can be memorials to your ancestors and visible to other researchers in years to come. This is another example of the blossoming interactive aspect of the Internet that is being applied to genealogy.
You can read more details about the service in Dick Eastman's newsletter at: http://blog.eogn.com/eastmans_online_genealogy/2009/03/commentary-footnotes-interactive-1930-census-and-the-great-depression-collection.html