My husband has pounded his fist on the sides of cars that come too close. He doesn't do it anymore; last time was 2 years ago and we were on a group ride right near our house. The driver was very much in the wrong and oblivious, but I convinced him his actions wouldn't solve anything.
My best one is the very first time I commuted from the house I live in now. I took a longer route home than I do now and at one point there's a road that merges into the road I am on right after you cross a highway with a light. There's a yield sign at the merge and it's busy with people coming from nearby medical offices and a hospital. I*knew* this one car wasn't going to stop, so I slowed down and he zoomed out, cutting me off. Of course, I anticipated it and I raised my hand, gave him the finger and screamed "a$$hole." The people in all the other cars around me started clapping!