The average cell phone charge uses a surprising amount of energy, even when it isn't charging a phone, just plugged into the wall. 5-15 watts may not be very much, but 5-15 watts, times each transformer you have plugged in the wall, times 24 hours a day, times 30 hours a month, and a 5 watt drain turns into 3.6kWh a month.
Most electronics have this phantom power problem. Anything that you can turn on with a remote will also have a significant stand-by drain, just listening for the remote's signal (ie DVD players, TVs, etc). Computers are another one; laptops aren't as bad (though they still have transformers like cell phone chargers), but desktop computers even keep the network interface powered up, to allow for WOL signals to be received. I have all of my desktops wired together on an easily reachable power strip under my desk, so whenever I'm not using them, they're physically cut off from the grid and have zero drain.
At my parents house, we found a compromise between the inconvenience of the switch on the power strip vs the cost of stand-by: We have just the TV and DVR on one strip, then the less used devices (digital TV tuner, CD player, VHS, etc) on another strip, which is usually kept off.