I subscribe to the very interesting QRP-L mailing list, and recently read an article by Michael Rainey, AA1TJ about an interesting radio and set of QSOs that he conducted. Michael's experiments are amazing and cool, but I think his latest effort takes the cake: he had a number of QSOs using a transmitter powered entirely by the power of his own voice. He calls the project El Silbo and it is a transceiver which uses a loudspeaker as a generator that drives a very low power transmitter. I find it interesting because it is obvious that if you tried to yell, you might be heard over a distance of one or two kilometers, but Michael was able to use the same power to communicate over distances greater than a hundred kilometers. Very cool.
I don't know why this percolated to the top of my consciousness today, but I thought I'd dump a link here so I can find it again. I recalled reading an article about the use of a particular special MOSFET device for use in crystal radios. It took me a tiny bit of time to find it, but here is a link to the Bob Cutler's QST article A High Sensitivity Crystal Set. In conventional FET devices, the FET doesn't conduct unless the gate voltage exceeds the threshold voltage. For a common power MOSFET like the IRF510, the threshold voltage might be 2 volts. For devices like the ALD110900A (datasheet here) the threshold is essentially zero. This makes a great variety of very lower power oscillators and amplifiers possible.
Cutler's crystal set design is innovative because it doesn't require an outdoor antenna or ground to function. It's a zero power radio with enhanced performance provided by this MOSFET device. I'm not the only one who thinks so either: no less a radio luminary than Wes Hayward, W7ZOI found Cutler's ideas thought provoking and interesting.
Addendum: G3XBM was looking at the same zero threshold devices. His page links to an application note for a 32 uw oscillator that can operate on a mere 0.5v supply voltage.
Addendum2: Ben Tongue has an interesting article on extracing energy received from a strong radio station to charge a supercapacitor, and then power a very low power amplifier that can be used to enhance the operation of his crystal set. It seems to rely on a micropower operational amplifier manufactured by Texas Instruments.