A couple of my recent experiments have been treemap visualisations using D3. Treemaps are useful for showing part-to-whole relationships when there are many data points and the data points can be categorised (countries by continent, stocks by sector and sub-sector etc). The Map of the Market is a nice example.
The interactive treemap I created of emissions and population is shown below. I think it does a reasonable job of showing the data, but perhaps it could be improved by dividing the countries into categories such as continents. I also wonder if side-by-side bar charts might be a simpler better way to show the data.
For more information and links on treemaps, see this article by their inventor, Ben Shneiderman.