I've been a fan of GoogleEarth since it was used to report on the London transit bombings. Photos and commentary from those directly affected were available within an hour of the attacks, all in a data-rich spatial context.
Mashups appeared as soon as Google released the APIs for GoogleEarth (and its online cousin, GoogleMaps). The latest (by Imran Haque) spatially embeds US Census data, enabling users to see population and wealth distributions, voting patterns, and more - all within geographical context.
By making the KML formating code open-source, Haque hopes others will begin adding additional data.
H/T to John Battelle for the link. Also, thanks to Imran for clarification on the spelling of his last name and the direct link to his site (see comments).