It's amazing that a name like that can grab the attention of so many people and trigger such a flurry of constructing. Is looks to me as if it's a way of driving a non linear device like an LED is such a way that it produces more light output than it would from a battery. Is it not just, effectively, a switch mode inverter to produce pulses of more current than the little battery, on its own, would provide? The Joule Thief doesn't claim to be an over unity device - it just seems to make LEDs look brighter. Why is that a "Thief"? Surely it's just proving to be more efficient (or is it?) than an approximate voltage source (a battery). Novel, but anything more? Has this been done on a large scale? Domestic lighting is so much more efficient using LEDs that it's hardly worth the bother but there would be many situations where PV / battery combinations could be cheaper for a given light output.