An article by Declan Butler in Nature Magazine mentions an interesting use of Google Earth – a computer programmer in Italy used Google Earth to find ancient Roman ruins:
Luca Mori was studying maps of the region around his town of Sorbolo, near Parma, when he noticed a prominent, oval, shaded form more than 500 metres long. It was the meander of an ancient river, visible because former watercourses absorb different amounts of moisture from the air than their surroundings do.
His eye was caught by unusual ‘rectangular shadows’ nearby. Curious, he analysed the image further, and concluded that the lines must represent a buried structure of human origin. Eventually, he traced out what looked like the inner courtyards of a villa.
On a more recent note, if you are lucky enough to have genealogy information about ancestors and their locations, specifically the kinds of records listed in the General Land Office patents (US Bureau of Land Management, which is unfortunately offline right now), to where you could really narrow their homesteads, etc., down to a very small area, and Google Earth (or MSN’s Virtual Earth), had high-resolution photos of those locations, you could, in theory, probably learn quite a bit and turn up some interesting stuff – maybe outlines of fields they plotted out, and farmed, or where the original structures, etc. were located.