Motorcyclists wave to each other in passing. It's normally 2 fingers in a V shape, pointing out and downwards, indicating "two wheels down" as a sign of wishes of safety and respect to fellow riders.
The wave isn't exclusive to motorcyclists though. Many other groups have their own waves. NA Miata drivers bob their headlights at each other. etc.
Rural Minnesotans greet other drivers they encounter by lifting their index finger off the top of the steering wheel and pointing it upwards, waving it once from left to right. The timing and cadence of the finger waving motion is important, and refined over a lifetime of driving.
New Jersey drivers use a different finger, usually with the whole arm out the window.
