Google Earth adds time to the mix.
With the advent of Google Earth, the surface of the Earth became accessible. By adding the element of time, Google Earth has just expanded this popular app to include all of world history. I would guess that with the API opened, there will be any number of mashups from environmental groups showing the shrinking rainforests to the transfer of wealth throughout history. I can see that a real time imaging of the hole in the ozone layer could be effective.
It should make some of Paul Allens mashup history ideas even more viable. I can see that geneology sites might be able to show your families history of movement.
In the future it might be possible to use your DNA fingerprint to map your gene pool to exact locations over time. Very cool.