There are now a range of energy saving plugs that you can buy:
An adaptor with a difference: plug your PC into the master socket and extensions into the side plugs, and when you switch off your PC, or put it in standby mode, the power will be shut down to the extensions as well.
Even better, the IntelliPlug looks for patterns of power usage rather than simply cutting off the power when it drops below a set threshold. Additionally, the Intelliplug has a basic surge protector to keep your computer safe.
Daily electrical savings including the savings from speakers, and printer equal 0.24KWh, and annually, 84KWh.