When you ride a bike, you cover a lot of ground. Unless you're riding underwater or riding across the Australian outback, you're likely to encounter a lot of people too. Some of them may even be cyclists, but most will just be walking or driving cars.
Here's the thing. The general public HATES us. They think we're a bunch of assholes who run lights and get in the way of their miserable commutes and don't pay taxes. Except for the last one, they're mostly right. So it would go a long way in cyclist-everyone else relations if we were to simply wave as we passed. Not like a "hi we're friends and I want to talk to you" wave, but more of a "I respect your right to be here and hope you would do the same for me" wave. Wave at cops, wave at pedestrians, wave at 60 year olds on hybrids. Especially wave at kids. When they hit 16, they really need to know that we're humans, and our lives have slightly more value than that text message they just received.
So wave. Let them know we're people too, and that we aren't all douchebags. We have a lot of bad relations to atone for. Don't be a douchebag.