The IPUMS-I (International) project (see http://ipums.org) is home to census microdata from all over the world. The number of countries and samples within a country is always changing as new data are added to the project.
This simple D3 example shows data availability within the project, in terms of number of household and person records contained in each of the samples. This example began with my curiosity about how many continents, countries and samples were represented, and how much data from each of these samples.
Derived from treemap example at http://mbostock.github.com/d3/talk/20111018/treemap.html - thanks to Mike Bostock!
This is work in progress.