First of all “free energy” is not an oxymoron. There are lots of easily used sources of free energy, like water going over waterfalls, sunlight, wind blowing, etc. Lesser known sources of free energy are gravity, the earth’s rotation, and magnetic fields. These sources of energy are harder to tap into and it is harder to get large amount of energy from them.
The first type of free energy device that I have experimented with uses an antenna as part of their power source. Supposedly Tesla’s self powered car used an antenna as its main power source. I have played with two of these types of generators. One uses a Tesla coil and the other just uses a bridge rectifier. I found two methods of obtaining more than one volt, but it was not enough energy to even light a LED! The first setup has the antenna going to the top of a Tesla coil secondary. The bottom of the secondary goes to a bridge rectifier. The other input to the rectifier comes form the ground and it can go through a smaller coil on its way there. The output of the bridge rectifier was about 3 volts, but dropped to 1.5 volts under load. A similar arrangement has the antenna and ground go directly to the bridge rectifier. That produced less than a tenth of a volt until I ran the output through a coil that was sandwiched between two large magnets. At that point it produced over one volt.
Magnetic generators use magnets and usually have electrical pulses to modulate the magnetic fields. If you believe that magnetic fields are not stationary but that they are always moving then all you need to do is to tap into those fields or modulate them so they reach a coil in waves to produce AC power output. There are several different arrangements of these generators. You can use two horseshoe magnets with pick up coils and pulse coils. You can use a long bolt with magnets on the ends and with many pick up coils and one pulse coil. Some of these devices have been patented and are proven to work.
Self propelled rotating generators are another source of free energy. To me if the generator takes a lot of power to begin with then it is highly suspect. There are generators that use a spinning wheel to produce static electricity. Then a high voltage oscillator is used to convert the static electricity into AC and then transformers can step it down to lower voltages that can be used to light ordinary light bulbs.
Motor Generators can supposedly produce more power out that what you put into them. They are very handicapped from the start because the motor is not efficient to begin with, so the generator has to be lie 200% efficient just to break even.