When people pull out in front of you forcing you to slam on your brakes (when there's nothing behind you)... then drive at a snail's pace, keeping you from passing them because they are driving at the same speed at the person in the other lane

People who don't know their blinker is on.
People who blast their car stereos so loud with crap making my windows vibrate (notice no real music ever gets blasted).
People who walk like they are taking a leisurely stroll in the park while you sit there waiting for them to walk through a crosswalk in a parking lot (I have just started giving a tap on my horn, which is loud, and when they jump and look at me with their eyes like saucers, I have a BIG smile on my face as I wave at them like when you hail someone down looking for you at an airport).
For me, it's mainly about rude and inconsiderate people.