Bikers wave to each other. My bro-in-law says he has NO idea why, but we do. And it's not a Mr. Henkie "HOW-DE-HO!" type wave - it's low, usually with two fingers. Basically it's exactly the same gesture as a "point to the lane" lane-change signal.
There have been hundreds of discussions on this: "Harleys don't wave" (everyone riding a Harley-Davidson is called Harley ;) ), "Squids don't wave", and so on. But there are a few "rules".
First, you don't wave in town. You're too busy watching traffic and trying to avoid getting run down by the cages to worry about waving. Second, you don't wave on a divided highway. The other guy's so far away he probably won't see you anyway.
But I met one of those trikes - you know, the ones that start out as a car and then they run out of metal and say "Oh hell, I've always wanted a bike...". Do you wave at them? I say yes, since they at least are not surrounded by metal and glass.
But that does bring up a point: All the other bikes I've seen on the highway are what we used to call "barges": Big motors, full fairings, big hard bags side and back, cupholder, cruise control... Hell, why not just add 2 more wheels and call it a car?
But to each his own, I guess. I wave at the barges...