A passenger who is constantly on your ass about your driving skills. He/she constantly cautions you to keep a safe distance from the car in front of you, checks that you're obeying the speed limit, and is always quick to point out your navigational errors.
Guy riding shotgun: "Could you slow down a little? We're dangerously close to exceeding the speed limit by 5 mph." Driver: "Quit being such a harassenger."