Motorcyclists. Yes, the hardcore Harley riders might joke about sport bikes, but often consider any old bike better than a car.
Hunters/people with trucks in rural areas. You acknowledge others when you are passing someone else in a place where other people are rare.
The Jeep drivers are trying to make friends so they have someone to pick them up when the Jeep will break.
Jeep. Wear good walking shoes.
Do school bus drivers count?
I noticed this phenomenon when I was a wee school lad myself and notice that it still goes on lo these many (many) years later.
LEAF owners wave.
Neither would the Nazi salute.
Everyone. I live in a rural area and we all do the finger lift from the steering wheel. One of my friends cracked up laughing when she visited, saying I now lived deep in ‘finger country’ 
And when I do go out of my area, it’s often on a motorbike, so am doing ‘the nod’ to other motorcyclists
Another vote for motorcycles. I get nods and waves all the time when I’m on my bike. Harley’s wave to rockets, rockets wave to Goldwings, it’s just about being on two wheels.
I haven’t figured out how Can-Am Spyders fit into the puzzle yet, but I think trikes probably get waved at.
That was actually one of the things my MSF teacher taught us, to wave at other bikers, regardless of what bike you vs them are on.
it’s “two wheels down.” it means you’re riding. and have so far been successful at not being killed by a cager.
Yep virtually all us scooterists and most motorcyclists wave (the two-down motion). We’re just acknowledging fellow riders and the love of riding.
If you have been killed by a cager and you wave, do other riders not wave back?
I don’t wave, but I do nod or slightly lift my crop to other people riding horseback. Then we look down our rather long noses at the peasants and commoners.
wow. the stick up your butt must have a stick up its butt.
Sticks are common. I have a small handmade free-range gluten-free organic tree branch.
But is it GMO?
The rule in the Australian outback is that the further out you go, the less you raise. In the near-country it might be most of the fingers. By the time you are in the true outback, things have become so laconic that a single finger, part raised, is about all anyone needs to express what is required.
In I believe 1989 or so I was driving my 1970 Datsun 1600 (I think they were called a “510” in the US) on a very lonely highway in outback Australia. They were an old car by that time and certainly not at all common.
It was the type of road where it was about an hour between signs of civilisation, and you could go all or half that long without seeing a single other car.
Then off in the distance I spotted an oncoming car that … could it be…surely not… another Datsun 1600? YES! I’m not normally particularly effusive but the fist pumping, screaming cheers, horn blowing etc started from the moment we each recognised what the other was driving until we could be heard by each other no more.
Aussies do it the fun way… W00t!!!
Datsun 240Z drivers do, then there ain’t many of us left anymore.
Porsche owners wave or grouse endlessly about people not waving as the brand’s been diluted to the point it’s just a car to some owners, or how some models are not a real Porsche (pretty much everything not a 356 or 911).
BMW Z3 coupe owners wave at a minimum, and often stop to chat, as there are only a couple thousand in the states"
I drive a 1987 Ford Tempo and people seldom wave yet there is one guy around here that always waves but I just give him the finger.
In rural Ireland, on B roads/Local roads, it is all but standard to wave to all other motorists.
I had a flat-screen VW beetle in the 80s (in England) and I’d often get the shaka hand signal from other beetle drivers. I wonder whether it was because of some association between Vdubs and surfer culture, or if it was meant to resemble the VW logo, or a combination of both.
Anyway, I would usually return it - albeit a bit sheepishly - if I could react quickly enough.