Here is a link to one more that caught my eye but have not had the time to try it out - maybe someone here who is to the electrical field may want to experiment with it - those who had experience with earth batteries would see why this link holds some credibility
http://www.mondovista.com/meyers/
another is about a gentleman who says his coil will increase energy if I remember right by about 5 times the input - but if true and if one could but these coils in parallel then even earth batteries may be a viable solution.
http://www.markorodin.com/