Well, there's nothing I like better than humiliating top scientists* and what's more, apparently this energy generator 'violates all the laws of physics', which is even more fun. So what would this involve? If you look up 'free energy generator' on Google you'll find lots of examples claiming to be just this - but overall it is a worrying concept.
The obvious problem is conservation of energy, one of the most fundamental aspects of physics. You have to be a little careful with conservation of energy - it does require a closed system, and we patently don't live in a closed system, so it's easy enough to get 'free' energy in the sense that the Sun is pumping vast quantities of it in our direction and doesn't expect to be paid for it. Similarly, a 'free energy generator' could just be a way to steal energy from someone else. It's perfectly possible to light a fluorescent strip light by earthing it near a high voltage power cable - but you aren't producing energy from nowhere, you are just acquiring (to put it euphemistically) a small amount from the power company. Which they probably aren't too enthusiastic about.
However, this kind of 'free energy' device is usually supposed to get energy from nowhere, so we are indeed talking breaking conservation of energy - and you might as well throw in perpetual motion, because the one implies the other. And that's a bit worrying because things have to come from somewhere... so where is the energy coming from? (You could also get a bit excited about the great German mathematician Emmy Noether's proof that conservation of energy was equivalent to symmetry in time, but that's probably too subtle to be useful here.) Energy conservation isn't always obvious, because energy can change forms and so become apparent where it wasn't obvious before - but in the end, this has to be one of the best established and easiest to support natural laws.
More dramatic still is the claim that this device violates ALL the laws of physics. I can't even begin to imagine what something that did that would be like. Of course, the concept of 'physical laws' is a little fuzzy. It really dates back to a time when it was assumed that God was in charge and these were the laws he laid down. A law requires the same thing to always happen in the same circumstances - in practice this can never be proven, but is a good assumption. However if all physical laws are broken, it would seem likely that the universe as we know it would fall apart, which doesn't sound too healthy. I'm not sure free energy is worth that consequence.
In the end, I refer the con men to that classic science fiction writer, Robert Heinlein who regularly pointed out TANSTAAFL. Not a shouty Scandinavian delicacy but: 'There ain't no such thing as a free lunch'.
* Actually there are plenty of things I like better than humiliating top scientists, this was just rhetoric.