This seems to be a nice tool to find campsites for free. At least in North America there are plenty of them listed. For the rest of the world it´s terra incognita more or less.
stick out this one, too: http://ioverlander.com/
You have to flag "informal campsites" and "wild camping"
It has a much better global coverage than freecampsites.
Frank / Motorcycle Vagabonds