I've the same experience quite a few times. Since it always happens on stretches of road where I need to keep both hands on the handlebars, I stick my tongue out at the driver.

My favourite cager moment was the day I signaled left and moved over a lane because the right hand lane had a disabled bus and massive tow truck further down. It was clear directly behind me, and in the lane I was riding over to on my left (and I am still signalling) and I continue over when a car appears and immediately starts honking his horn. Luckly the road is smooth and as the moron rolls past I add my middle finger to my left signal, and stick my tongue at him. Fool was just upset that he had to mind the rules of the road and pay attention to other users of the road.