FlowingData highlighted this data project from WallStats showing how US tax money was spent. The US government being the sprawling beast that it is has an incredibly complex budget, and this visualisation not only makes it accessible but pulls the reader into exploring it.
It has to be good. It even had the American queen of home decorating and entertaining, Martha Stewart, talking about it. I also love is that by using Zoomorama, they have leveraged a printed poster online, simply but quite effectively.