Like Pedal Wench, I need to vote for both left and right. I use whichever one will be the most visible to whomever I am signalling.

I will use my right arm when there are only 1 or 2 bicyclists behind me. If I'm signalling to a driver across the road, I'll also use it being sure I can see the driver's comprehending eyes.

Whenever there is a larger group of riders or cars behind me I use my left arm but really am pointing with it over the top of my head. Seems like it is more visible because my hand is high up over my head.