For me it's blinkers aka turn signals/lights that are built into cars. Too many either rarely use them (if at all) or have this idea that the blinkers are for their benefit (as opposed being for the benefit of others).
Hardly anyone uses them inside residential neighborhoods, I guess because there are no stop-signs or real intersections. There will be times when I'm walking the dog around the neighborhood and a car is coming. It would be great to know if they are going to go straight and pass me or make a turn right in front of me, so I can plan accordingly. Of course they will come along, doing 35+ in the residential neighborhood, turn right in front of me without using a blinker, and sometimes even honk, like it's my fault that they almost ran me and my dog over...
Other times I'll be in a left turn lane at an intersection directly across from someone who is in a lane that can either go straight or turn left. In that situation, I would look at their blinker to determine what they are going to do. If we are both turning, then when the light turns green, we both turn. If he is going straight, then upon the light turning green I will proceed into the intersection (with my left blinker on, and while keeping right) and wait for him to pass me before I complete my left turn. But if the person intends to turn left, and is simply too lazy to use their damn blinker, the result will likely be them practically turning into my car in the middle of the intersection.