Adam Schneider’s GPS Visualizer is a collection of web apps that, among other things, converts GPS data into KML files. Now this workhorse learns yet another trick, letting you create overlays for areas you specify from USGS, Landsat or Canadian data, including topological and political maps. Behold the GPS Visualizer KML Overlay input form.
(USGS data availability (via TerraServer-USA.com) is spotty, Adam says, so for a sure result, experiment with the other data sources.)
[2006-02-24 00:20 UTC: Post edited to atone for my ignorance of the fact that GPS Visualizer in fact does far more than just cater to Google Earth. The question, rather, is what doesn't it do? Check out the geographic calculators, many of which output to KML. And don't forget the geolocator. And here is some eyecandy.]