Perpetual motion is the alleged claim of being able to extract useful energy by violating one of the laws of thermodynamics or tapping into a scientifically unknown source of energy. Perpetual motion doesn't exist.
What is not perpetual motionEdit
The Heisenberg uncertainty principle forces the product of the momentum and position of a particle to have an uncertainty above a constant. Thus, knowing a particle is in a box forces it to have momentum. The particle will thus never be still, but this isn't perpetual motion because there is no way to extract useful energy from this phenomenon.
Superconductors: Even assuming the current lasts forever, this is not a perpetual motion machine because any attempts to extract energy from current in a superconductor would lower the amount of current flowing, like discharging a battery. Similarly planets orbit indefinitely unless something outside interferes, but extracting energy from an orbiting planet reduces it's energy of motion.
Nuclear fusion: It's thermodynamically allowed, but very hard to do. However, this could very well become a viable source of energy within a generation. But this doesn't create energy, it converts mass into energy.
Any machine that claims to get energy out without using something up.
Steorn, not Stormfront but just as ludicrous (in a much more benign way).