An Autowit Supercap 2 car battery jumpstarter.
Got it yesterday. Neighbor has a car that wouldn't start, battery was so dead he couldn't even lock the doors and none of the indicator lights lit up. It was so dead we couldn't even get it to jumpstart from my car. Just by coincidence I had ordered this thing early in the week.
I charged the capacitor from my USB port this morning (took about 2 hours) and then went next door to try it out. Was sure it wouldn't start a car with that dead of a battery, but it fired right up. Apparently the surge current was enough to overcome whatever resistance was in the battery