Tesla free energy devices.
I saw an instructable on one and read the comments. People were getting very small amounts of voltage from a particular design, and of course this is not impressive or practical. The device used a simple circuit and a very small antenna. Could be carried in your pocket. However, if what he said is to be believed - one can increase voltage by placing a larger antenna at a higher elevation (such as a roof) and running a ground wire - along with more of the same circuits connected in parallel or series.
Someone responded that this was "stealing radio waves" and could cause a "radio black spot". Except - this device is supposed to work in the middle of no-where - where there are no radio waves to be picked up.
I'm intrigued. Do I part with 40 bucks? That is the question.