These are great little gadgets and have a variety of ways of working.
The simplest seems to have a master socket for the PC for example and the other sockets power down when the PC is switched off.
The second type of solution is a remote control. Simply turns off the sockets via a remote control when you leave the house.
Lastly, the cleverest form of solution seems to be the products which automatically detect when the device is in stand by mode, or is no longer charging and automatically shut off the power.
These devices will not only save you money but can give you more peace of mind knowing your household devices are no longer drawing power. Lets be honest though, we could just walk to the plug socket and switch the appliance off at the wall. However, with the number of appliances in the average home, these devices will make your life easier and safer.