Energy harvesting
Powering electronics from whatever energy happens to be lying around — light, heat, vibration or radio waves — instead of a battery.
The budgets are tiny: a stamp-sized indoor solar cell yields tens of microwatts, a thermoelectric element on a warm pipe about the same. Not enough to run a processor continuously, but enough to charge a capacitor and wake up for a few milliseconds at a time. EnOcean, spun out of Siemens in 2001, made it commercial with wireless light switches powered by the press of the button itself.
The software has to assume power can vanish at any instant. This is called intermittent computing: state is saved continuously to non-volatile memory such as FRAM, and the program must resume where it died rather than starting over. The brown-out detector stops being an error path and becomes part of normal operation.