Last week Google added quite a bit of new featured content to their Google Earth application. This includes historical maps from the Rumsey Maps Archive, two layers that deal with Africa, and imagery from the European Space Agency. Also the UNEP Atlas of our Changing Environment is a fantastic tool for exploring how human activity impacts the Earth.
By the way, the latest version of Google Earth now allows the adding of image layers. In the example below a map of Portland from 1897 is overlaid on the map. By using the opacity tool one can easily compare the information on the map to the present day information. Jason Kottke did the same with a historical map from 1836 and combined it with the 3D buildings view to produce an interesting image of Manhattan.