I love clever ways to visualize data, especially when they can really make the trends pop out to the viewer. What are the best visualizations you've seen? What have been the most innovative, the most illuminating, or the most impactful?
One of my favorites, which you might have seen during the opening ceremony at the Summer Olympics, is the spiral graph of global temperatures over the last century: http://www.climate-lab-book.ac.uk/spirals/
And, of course, the always fascinating Hans Rosling is the master of data visualization: http://www.economist.com/blogs/babbage/2010/12/data_visualisation
What are your favorites?