Good grief. Don't get me started on snake oil
I for one believe that most, if not all of these don't have a chance at working (leaning heavily towards the all). For any of them to work they would need to fulfill one of two criteria.
1. Be an open system where energy is entering the system from an external source. E.g. PV, Wind, etc.
2. Break the laws of thermodynamics. E.g. ... ... I have no examples of this
If these systems really work, they would need to fit into the first criteria as it is accepted that the second one is a load of garbage (read BS). Therefore an inventor would need to explain where the external source of the energy comes from and I've heard it all folks. Everything from heat through to zipons (lol @ zipons). This external energy would need to be measurable is some way and a common excuse is that the energy is unmeasurable by conventional equipment (see cold electricity lol)
An invention that, IMO, has promise is currently being "marketed" by a convicted scammer by the name of Dennis Lee (surely everyone has heard of him). This one is probably the most well known of the snake oil, Google his name if you want a bit of a laugh. Some other unfortunates share his name, but you'll get the gist of it.
More food for thought... I read somewhere that Zero Point energy has been confirmed by NASA (or some other institute) as being real. Problem is that it is random and uniform in nature so extremely difficult, if not impossible to leverage.
Anyway, I'm pretty sure I'll get flamed, corrected and persecuted for this so go for it guys.