It is time to talk about spatial analysis. Many open source GIS software have at least some analytic capability--more functionality is being added frequently. Earlier, I showed a simple map of wifi locations in New York City using QGIS. Let's take a look at the density or in this case area surrounding these points. Since I have had trouble with kernel density, let's use Thiessen/Voronoi polygons. Interestingly, these are only available with an ArcInfo license in ArcGIS, which is extremely expensive. I am not going to compare results here, but let's see what the resulting map looks like. The lighter/whiter the color the less area between wifi locations and the better the wifi availability. (Of course I don't show whether the wifi locations are free or cost-based on this map). Not bad for free data and free data analysis! I used the nifty vector transparency plugin from QGIS so you can also see some of the land cover.
Click on the map and a larger version will appear.