We use treemaps a lot in strategic workforce planning and analysis, and sometimes people new to them struggle to understand them at first.  They are mostly used in financial analysis, but the Information is Beautiful has a nice one called the billion-dollar gram, which shows how the relative size of issues can be clearly illustrated using a type of treemap:

Note that it uses different colors as well as different intensities to provide a third aspect to the issue.  Lovely.  Anyone else got some good treemap examples?