The poor man's MPPT. A self-oscillating circuit that provides dc-dc buck conversion as well as automatically maintaining near-maximum solar panel power. It is not a true MPPT as it does not actually "track" the maximum power point.
It is simple and robust, requiring only the following components:
P-channel MOSFET
2x resistors
1x capacitor
1x trim-pot
1x diode
1x NPN transistor
Optional: zener diode and inductor.

Ideal for small portable solar kits or off-grid prepping.