It’s a fun engineering challenge. Weird energy harvesting tech mostly has applications for sensor networks. Some of the new generation of bluetooth chips have ridiculously low power consumption – so being able to deploy them without a battery somewhere without maintenance is occasionally useful.
Some currently used technology are piezo energy harvesting from mechanical vibration, low-light solar, and thermocouples.
My approach is usually to think around the TPL5110 and a pseudocapacitor. The TPL5110 is a timer that has a current consumption of 35 nA and can operate down to 1.8V. Every 2 hours, it would activate an ATtiny10 that can operate in the microampere range. That chip (very quickly) measures the voltage on the capacitor relative to a reference and decides whether it has enough power to “do the thing”. If it does not, it signals to the TPL5110 to turn itself off for another 2 hours to let more charge build up.
If it does have enough power, the ATTiny10 either “does the thing” itself or switches a MOSFET to activate another system or whatever. The “thing” can be to use the power stored in the pseudocapacitor to charge a battery for a short time (e.g. around a second), if you want. Afterward, the system goes back to sleep until it has a relevant amount of power again. However it’s often a battle to outpace the self-discharge of a lithium cell, so having the system “do a thing” without a battery present is often better.
This does result in practical stuff sometimes, especially when using low-light solar. Besides sensor networks, you can for example manufacture replacements for tritium indicator lights this way that only activate on at night. In my experience, an SMT indicator LED is quite visible at night with under 10 uA of current. I have a series of ridiculously overengineered indicator lights that stick to the top of doorframes so I don’t hit my head on them at night (I am quite tall, and live in a traditional home in Asia).
Incidentally, I tried building a resonant circuit at 60Hz and was able to pick up a few mV from nearby fluorescent lights – not enough to use. I used a ridiculously large coil of wire that I happen to have lying around. A more fun trick is to use LEDs as their own power source – during the day they work as tiny solar cells, and that lets them flash occasionally at night :D