In August 2006, real estate search site Trulia had 609,000 visitors. Five years later, there were 27 million. Trulia's most recent visualization shows this growth (bottom bar graph) and where people are searching for homes (map). Press play and watch it go. It's pretty much population density, but for me, the method is more interesting than the material in this case.
The grass aesthetic is kind of nice. It looks like you have a one pixel blade of grass for each zip code with a significant search count (If only there was something to provide scale...), and where there's more search there's more grass.
I also like the relatively simple tech behind the graphic. We usually see animated and interactive maps generating everything on the fly, but the maps and bar graphs for this are pre-generated for each month. Then each image is displayed one after the other chronologically like a flip book.