I created a transparent hex grid overlay for Google Earth. The hexes should be sized to 6 mi, but your results may differ on other parts of the globe. The overlay should tile with itself, so if you need to cover a bigger area, just copy it as many times as you need.
The Google Earth File can be found here:
The file will preview in Google Maps in your browser, but you'll want to download the file and open it in Google Earth in order to modify/move the grid.