Around here, I'm happy to say most bikers, from harley guys to tourers to sports, and even scooters (biker wannabes,) wave to other oncoming bikers. Not sure if there is any reasoning behind it but the most common wave seems to be a 2 finger wave (lazy peace sign?)
Although there are always some who obviously think they are to cool or whatever to wave, I don't hold it against those who don't as they may be clutching or may just not be comfortable taking thier hand off the bars. I tell new riders not to feel compelled to wave until they are totally comfortable doing so.
Hand on the helmet seems to be universal cop signal. Although the finger twirl, although used for "start your engines" in group rides, works in context.
Something not mentioned, and maybe I'm the only one who does this but, when not readily obvious to the oncoming rider, I'll tug on my raingear to try and communicate that there is a shower (or snow as the case may be) ahead. This may sound stupid but in our neck of the woods, it can threaten all day but never get wet, or be sunny on one side of a mountain and raining on the other, or... you get the idea.
In group riding, I'll use foot signals to indicate changing tracks within the lane (I reserve signals for actual lane changes or turns.) Everything else is pretty well documented except...
Stand on your pegs and point at your a$$ to say you have to take a dump (i.e. - a tree won't cut it!)