Here's a great post from Jeanne Kramer-Smyth at Free Government Information (FGI)
"One great way to get your head around large government datasets to view it using Google Earth. I went on a hunt for the most interesting, striking and geography based government data sets currently available in the KML format used by Google Earth. There is a large gallery of tours and layers available from Google Earth's site, including some based on government data - but I wanted to look beyond them.... Check out more of the entry at Free Government Information (FGI)!