I must be nuts - or OCD - (probably the latter) because I just don't seem to be able to leave these border files unfinished. The last file I published was the US States file. Now, I've got the US Counties file ready for download and it was even less of a small feat than the state file was less of a small feat. [Translate: when I called the state fall "no small feat" I really had absolutely no idea just how difficult the USGS could make it to extract their data into a KML file. None at all. Now I do!] [A free Google Earth file of this route is available at: US County Borders.]
All of the CONUS counties are included as well as Hawaii, Puerto Rico, Virgin Islands, Guam and the Northern Mariana Islands in one Google Earth KML file. Well, let's say, that all the links to each of the state counties are included in one file - believe me, this data is massive! Poor old Google Earth will choke on the entire bunch at once so the map opens and you need to select the check box next to the state that you'd like to see.
It will then download all that state's data and make it visible. You can have multiple states open at once but as you do GE get's slower and slower to respond. Strictly for convenience sake I've rearranged all the states into the major/accepted state areas - like pacific, mountain, new England etc.
If you'd like to save a particular county, state or area you can right-click on the file portion and select "save-as." If you'd like to alter the color or transparency, right-click and select "properties".
Hope this is useful to someone... besides me!