Energy is free. There are plenty of technologies that can produce practically limitless energy for very little cost (eg. Cold Fusion, Vacume Energy). The only thing is that governments and big corporations have been actively suppressing this technology for their own gain.
I assume by "limitless" you actually mean "more than we human's could ever use" because the idea of unlimited energy breaks a fundamental law of physics called the conservation of energy. Cold fusion has never been achieved in any verified experiments. Vacuume energy, as far as we know, can not be harvested for 'free energy'.