As intruduced on the Free Energy page, this term is a recent label for one of the oldest (and still very successful) scams in history: The sale of a perpetuum mobile (PM).
This article tries to collect such attempts worldwide. Please note that use of the term 'free energy' is just one attempt to camouflage the fact that a PM is to be sold. Other common catch words are 'cold fusion', 'zero-point energy' and a plethora of senseless hogwash to fool the gullible, preferably those with money. Further, such machines are often called converters, pretending energy is only transformed (which is possible within the bounds of the second law of thermodynamics).