I'm surprised nobody is using a device based on a crystal radio to tap into the 'free' radio waves all around us and generate some heat/light ;)
In one MythBusters episode, when testing "free energy" they actuall succeeded in generating a small amount of voltage (I think about 1/2 a volt) with just a long wire connected to some sort of circuit.
What I couldn't understand is their reason to mark the myth as "Busted", rather than "Plausible". Maybe with a small room with 220 wires connected to these devices, you could generate yourself 110 volts of truely free energy. But maybe this isn't a technical possibility.
However, energy like this has to be coming from somewhere (probably radio waves), so it's not really free; just "leeching" electricity off of local radio stations.
However, the idea of "free energy" (when interpreted as energy which is provided to you for free) is very plausible - in fact it exists right now, in the form of solar panels, etc.
Steorn's alleged "free energy generator" may be nothing more than a solar panel that operates not on light, but some other form of energy. Since not much information has been released about the operation of their device, it could be anything (provided, of course, that the whole thing is not a hoax).