From the About page:
Hotmap shows where people have looked at when using Virtual Earth, the engine that powers Live Search Maps: the darker a point, the more times it has been downloaded.It is a pretty cool idea. The heat maps clearly focus on high population areas, roads, coastlines, rivers, country borders, and other items of interest.
Seems like there could be a bunch of unusual and useful applications here, especially if you take into account time series (that people look at some map tiles after looking at other map tiles).
Danyel Fisher at Microsoft Research has two articles on the project: "Hotmap: Looking at Geographic Attention" and "How We Watch the City: Popularity and Online Maps". The articles are light reads, plenty of pretty pictures of heat maps.
As the second of the two papers mentions, there are some amusing examples of where people direct their attention. Here in Seattle, a "small, very bright point on the shore of Lake Washington points out Bill Gates' house."
For related work that uses GPS log data rather than map search log data, also make sure to check out "The Microsoft Multiperson Location Survey" and "Predestination: Inferring Destinations from Partial Trajectories".
Update: More on Hotmap from Matthew Hurst, Todd Bishop, and directly from Danyel Fisher.
Update: One month later, the NYT reviews a commercial device called the Dash Express that apparently "broadcasts information about its travels back to the Dash network", allowing users to "warn each other through the network [anonymously] the second they hit a traffic slowdown."