People who seem unable to navigate roundabouts and indicate their intentions at the same time.
People who ruin a NSL section of my scenic route home by driving at 45 mph and hammering the brakes on gentle corners. These are usually the same people that sail past the 30 zone signs at 45.
People who MUST overtake me, while I'm doing roughly 70 in the inside lane on an empty motorway, only to cut back in front and slow to 69.995 mph about a foot in front of me.
In fact, other people on the road in general