Worst thing I ever did was after a passenger screamed at me from 2 feet away while overtaking me in a pick-up truck with construction equipment in the bed. If you've ever been barked at by a dog right in your ear, this was a very close second. You know the kind, where the dog doesn't bark until they are right up next to you, then as the car goes past you at 40 mph, the dog leans out the window and "B A R K!!!!" I've gotten both wheels off the ground from one of these before! Anyway, back to the story. This was when I was very new to commuting and hadn't fully realized how dangerous and mentally ill many people are. I decided right away I was going to catch them and spit on their car. It must have taken me three minutes to catch them in traffic. I unleashed a huge spitball through the open window and it blasted the passenger-side of the dashboard (I was probably aiming for the guy's face). I can still hear the wailing and groaning when it hit. They passed me a little while later and screamed obscenities at me. Thankfully, they didn't run me over or throw a hammer at me, nor did I end up splayed-out on the hood of a police car being arrested in front a few hundred cagers. Just another example of God's grace, I suppose. Repent and do better tomorrow.
I've seen plenty of road rage directed at me before and since, some more violent, but the most I'll do now is follow them or circle their car once while parked in traffic like a shark.