I had a devil of a time finding simple GeoJSON and KML boundary files for US counties and states. Eventually I realized that I could get shapefiles from the United States Census Cartographic Boundary Files and convert them to GeoJSON and KML formats using the MyGeoData vector converter.

The result is the following set of boundary files. Since copyright protection is not available for any work of the United States Government, you should all be free to use for any purpose. The Census Bureau does request to be cited as a source.

These files are available in various resolutions and are all derived from the 2010 census. The 500k files are the most detailed, but also the largest. The 20m files are the smallest, but at the cost of some dramatic simplification. The 5m files fall somewhere between the other two.

If the files you need are not here, don't be afraid of going to the source and converting them yourself!