Accidental/unintentional near collision situations: A quick holler for the idjit to wake up and watch where they're going, then I hammer the cranks for a while and burn off the excess adrenaline.

Intentional horn-blowing obviously trying to hit me situation: Cleat to the tail lights. I also have some short track-spikes screwed into the front of my left shoe. (They're MTB shoes with 2 tap holes for putting in the included football style cleats, but they accept track spikes, too.) They're good for gouging into someone's paint if you're in a situation where you need to fend off with your foot to keep from plowing into someone.