NeighborhoodBase includes maps and data from a number of city agencies, plus census data. All data is available for aggregation at a range of geographic levels (ie, wards, census tracts, etc). A great source for data and maps about life in Philadelphia.

Here are examples of different ways of projecting the Earth's surface and a brief discussion of the pros and cons of each of the methods.

The US Census provides thematic maps at various levels of geography and for many data points.

Allows users to store, tag, and share locations -- based on google maps, but does interesting things. Users add photos of places, and lots of other tagged info about the places.

Historical Maps plus GIS from the David Rumsey Collection.

While many businesses were in the path of these storms, some were affected and others were not. Knowing where affected businesses are concentrated can guide efforts by Federal, State, and local agencies, service organizations, and entrepreneurs to respond to the storms and rebuild the economies of these areas.

In this interactive web site, you are able to select a city or metropolitan area and view the location of high-poverty census tracts and observe the growth of high-poverty areas over time. In addition, you may view the changing demographics of the population. All of the maps here are based on U.S. Census data from 1970 through 2000. The data are summarized at the neighborhood level, using Census Tracts as proxies for neighborhoods.