A couple months ago I began a project to import all 50,000+ USGS Topographic Maps into Google Earth. This is a huge undertaking that will likely take me several more months to finish all 50 US States. These maps are intended to provide a free alternative to expensive commercial products that often cost $80 or more per state or charge monthly access fees. Plus you get the benefit of using the topo maps with Google Earth.
The Topo maps are saved as individual KMZ files which must be downloaded in their entirety before they will be visible in Google Earth. Most of the maps are less than 5 megabytes, but some are as large as 20 megabytes so the speed at which the maps load will depend greatly on the speed of your Internet connection.
Currently, topo quads are available for Arizona, Nevada, Colorado, and New Mexico.
Learn more and download the KML file at: http://www.gelib.com/usgs-topographic-maps.htm