Lighting is a very big user (and waster) of energy around the world.  To see just how much of a burden lighting is, just check out Google’s catalog of images of the earth from space at night.

Lights left on when you leave the room?  A fan?  The rule of thumb is that when you are gone from a space for more than 45 seconds it will save energy to turn electrical items off rather then leave them on (there is a ‘surge’ burden associated with switching things on which accounts for that 45-second ‘breakeven point’).  But unless you are REALLY coming right back, take a moment to turn things off.  It is both effective as an end-saver of carbon emissions, and as a ‘waker-upper’ on your general effort to change habits.