TerraServer-USA data served to Google Earth - This is a KML Network Link (which acts somewhat like a plugin or extension) that allows you to view various alternate background overlays within google Earth. View Landsat imagery, Blue Marble data and even USGS topos (DRGs)... an awesome free plug-in for Google earth. See http://www.gpsvisualizer.com/misc/kml_network_link.html