Some recent news:
Combining OMRON’s electret energy harvester with a Holst Centre/imec power management IC, it can convert and store energy from vibrations in the μW range with high efficiency to the driving voltage of general sensors.
μW .. say 10 μW then with 3 V you have a current of 1 μA and if you believe Power saving techniques for microprocessors that should be possible in sleep with the watchdog disabled (0.1 μA) but not with the watchdog enabled (4 μA)
But .. just wondering why not using a battery? Is it because you think it saves energy?
The embodied energy used to produce these devices is for sure larger than they ever will generate, see the articles on http://www.lowtechmagazine.com or http://www.lowtechmagazine.be, for instance The ugly side of solar panels.
Is it fun? Yes! But do not foul yourself thinking this is saving energy.