I typically give a quick honk when someone isn't paying attention (sitting at a green light & fucking with their phone, etc). I normally give them 3-4 Mississippi's and then hit em with it.
I broke out a rare rage-induced honk on someone a few months ago. It was lightly raining and someone was going 20 in a 45 (strike 1). I followed them to a T-intersection that was light-controlled. They were turning right and didn't actually turn right on red (strike 2). Once the light turned green, they started going and then got about halfway through their turn and abruptly came to a near-complete stop even though there was absolutely nobody on the road except me and him (strike 3).
I had enough and laid on the horn as I ripped by them. They absolutely shouldn't have been on the road if they were that scared of driving in the rain.
I'm convinced they were either 80 years old, drunk, or somewhere in between.