The power consumed in Britain while electrical devices are on "stand-by" produces a million tons of carbon dioxide a years, according to the Independent.
(The worst culprit has to be the Sky Digibox, which cannot be turned off without removing all sources of power. The "off" control disables video and sound output and changes the colour of the front LED but leaves all other hardware running, amounting in as little as a 1w power saving over the box being fully on. My solution? Switch it off at the plug.)