Solar panels. Having several solar panels and a small battery bank would give vital minimalist energy needs. To have the small amount of electrictiy to run your refridgeration and heating system for a period of time every day would save you from expelling so much energy and time. In a shtf situation there will be so many priorities, but not having to spend huge amounts of time and energy on providing refridgeration, warmth and shelter will allow you to spend more time on gathering food, water and other necessities. Small amounts of electricity also can contribute immensely to counteracting boredom (tv, radio games, and computers if available) . It allows the ability to gain information. Having the ability to turn on a light at night would boost morale tremendously.
I know that the question stated an item that was brilliant and not commonly thought of. I am stating solar panels because although everyone has thought about them, very few actually have them.
With all due respect to Tater, having the knowledge of where to obtain a solar panel is vastly different than having one. When the shtf I want to spend my time and energy on whatever emergency that must be dealt with. To have to try and acquire and implement a solar energy system while everyone around you is scrambling for whatever resources they feel are important, would put yourself behind.
a small solar system is simple, you need 4 items
solar panels to gather energy
charge controller to regulate energy to batteries
Battery bank to store energy
power inverter to create 110 volt ac
It is that simple, even if you do not want to set up the system, start buying the components. Start with one solar panel, then buy a charge controller (buy big enough for expanding the system), buy one or two batteries (agm deep cycle) and lastly a power inverter (again buy bigger for future expansion). Now that you are set up you can add solar panels and batteries until you are satisfied.