Trulia Hindsight is an interesting mashup that shows the grow of populations in the US over time. Their intent is to document the creation of every house in the US. Using this data and Virual Earth, they have created animations that place dots on the map when houses were built in any particular area. The map features a few pre-built animations, like “Urban Decline of Detroit” and “Formation of a New City”. You can also use the search engine to find a particular area of interest and watch the houses being created over time. The data seems to be the most robust in their target areas – which is to be expected – but there are other areas which have good data. Our area does not, but the town in North Carolina where my father grew up has plenty of data back to 1880!