The most offensive thing is when people throw objects. Personally, I have been used as a target for bottles and eggs. I've been honked at incessantly as you describe too, but my pet peeve is when people pass from behind, pull in front, and then brake and make a right turn in front of me, forcing me to brake, shift, etc.

At the very least, I give a good, hearty yell at the drivers. It has to be loud so that they can hear it through their windows. I usually swear so they know I am not pleased. When a driver has expressed displeasure in my rate of travel, and we are on city streets, I usually do what I can to catch them at a red light, stop next to them, and smile right in their face. I used to carry a golf ball to throw at people, but it was never very practical.

Incidentally, I only have a temper when I'm on my bike. I'm not sure I've ever even been in a shouting match. On the bike though, I feel a responsibility to remind drivers that the two-wheeled, pedalled vehicles are carrying humans too.