How much electricity does a phone charger use?

I think this is intended as a relative question, so here's a relative answer that may answer the real question:

Although all (cellular) phone chargers do use a little energy when idle, it isn't a lot. The most modern chargers are Excellent performers. One exception here- If the charger is warm to the touch when the phone isn't connected or charging, it is likely an older transformer type, which is larger and heavier than modern ones. Warmth=wasted watts.

If you are interested in saving energy, and the phone charger's role in saving energy, be sure to get all the "fat ducks" first- the ones that save tens or hundreds of times the amount of energy involved with the phone charger. Switch to CFL light bulbs, buy energy star appliances, heat less, cool less, drive less, insulate, weather-seal. The phone charger pales in comparison, even though you can get an impressive figure by multiplying a few Milli-watts by millions and millions of chargers.

If you are really serious about every last Milli-watt of power you use, then by all means unplug when not in use. (Do protect you outlet though, or you will have the expense of replacing it as it will wear out fast. Use a multi-tap outlet, or an extension cord, both of which are cheap to replace, or you can buy a switched outlet that plugs into the regular one.