http://a.tiles.mapbox.com/v3/modilabs.map-wnigdvsl.html#12/40.721/-73.880
This map, created by Columbia University’s Engineering School does an excellent job of showing the estimated annual consumption of energy by buildings in New York City. Right from the start we’re able to identify that red mean more energy consumption than green and are able to calibrate all the color shades in the middle accordingly.
The layout of the map is simple and easy to understand and yet it contains a remarkable amount of information, if the viewer is so inclined. If you click to pull up the interactive version of the map, you can click on each block and it will give you the energy consumption stats for that area. More importantly it gives us the breakdown for energy consumption which is arguably more relevant. We can see how much are building expends on heating, cooling, electricity and hot water. This helps us answer questions about how we use energy in the year. Some of the results are unexpected, when we find that heating takes up more energy than cooling, given how much we run air-conditioners in the summer. The information is also relevant to landlords or real estate developers who might own blocks of property in the same area and come up with collective plans to cut down their energy consumption.
There is also the element of personalization. We can easily navigate our map to the street we live on or the building we work in and get the figures for our energy consumption. We could also use this information to compare different areas or buildings in the same areas that have differing energy consumptions, leading us to investigate the causes for that.
Via NYTimes Green.
Omar Bilal Akhtar 
I also think this is a great visualization, really illustrating how the relevance of the data and the detail of the geography can create a really useful and informative graphic. One addition I would like to see, too, would be the option to shade the map by individual consumption categories, corresponding to the pie charts available. While heating is generally the largest source of energy consumption, this could provide useful policy information, such as targeting certain buildings/areas for retrofitting programs to prevent heat loss. It would also be worth analyzing whether those buildings that consume a lot of energy for heat even use air conditioning in the summer. Aside from this, the design could use some minor tweaking for ease of use (such as resizing the detail window so that the pie chart does not generate a horizontal scroll bar). Overall, though a really powerful graphic.