Can you buy, somewhere, a USB cord that has an electronically controlled cutoff switch in it? Ok, I know how to make one with a P-channel FET and a resistor. It's just a simple product idea for people who have, for example a Raspberry Pi and want to be able to turn it on and off under software control. (the "on" part would obviously have to be controlled by something else- maybe an IR remote- or another Pi).
I found some USB hubs with physical power switches on each port, but many of these also have active current limiting to 500 mA per port, which is unfortunate since the RPi can easily draw over 500 mA in normal use. Not sure if they have just a polyfuse, or a real current-sense circuit. For the prices I've seen, probably the former.