Bad driving etiquette. I can't think of any greater source of stress in my life. People who don't use their turn signals, people who drive in the left hand lane (i.e. too slowly, and then refuse to get out of the lane, even when people start pasing them on the right), people who don't know when to get out of the way of other drivers merging onto roadways, people who drive with their high beams on... I could go on and on, but I think you get the point.