Drivers who don't acknowledge that you pulled in to let them through a narrow bit of road, especially when the obstruction is clearly on their side; People who bump into you but don't apologise; People (usually men) whistling in shops, museums, even libraries; People who walk straight to the front of a queue, ignoring us folk who have been patiently standing there for ages.
Oh, and the thing that makes me murderously angry when I put the 'next customer' bar behind my shopping on the supermarket conveyor belt and the person behind me moves it forward until it's crushing my shopping. It's not as though I place it far behind my shopping, just about two inches, but for some folk that's obviously too far! I always move it back and say very politely 'Sorry, but if you're going to crush my shopping then would you like to pay for it too?' Sarcasm is wasted on most of them though, but it makes me feel better! Strangely enough, DH has a tendency to vanish when this happens, he runs off and hides in the car!