The second car I ever owned was an MG Midget. There weren’t many on the road, but we all waved to each other.
Lately I’ve been driving Jeep Wranglers and there’s a lot of waving going on again.
In between I’ve driven Toyotas, a Ford, an Accura with nary a wave.
What other cars wave?
Corvette - Save the Wave
Mini Coopers.
In Albany NY there’s only a handful of my car - Blue Raspberry Honda Fits. We’ve all been waving to each other. 
1987 Ford Tempos, I kept trying to wave to those guys but they just gave me the finger.
I wave at other (Subaru) WRX drivers and get a return-wave most of the time.
G’on, it’s safe. And short.
Hippies in VW microbuses wave…either that or they are just trying to clear out the smoke…
My gf drives an Imprezza. She laughs at all the Wrangler waving.
A friend and I have a tradition on road trips–when we’re driving on backroads or through National Parks we wave at oncoming cars. If the people in the other cars wave back we cheer. If they don’t wave back we shout “Those bastards!”
BMW 2002’s. Sometimes old e30’s. Probably Volvo P1800’s, but I’ve never seen another one on the road while driving it. 
Rural areas, everybody and they all wave back.
I had a B (MG) in the 70s. First car I ever bought with my own money. Fellow drivers, few though they were, would always acknowledge one another.
My dad bought one of the early VW Beetles. He and my uncle had to drive up to Traverse City, MI to pick it up off the ship from Germany. VW conducted a class at the port for anyone getting one of the cars, since there were no English owners’ manuals and any writing inside the car was in German. I was only an infant at the time, but my mother told me there were VW Beetle clubs for enthusiasts and that my dad and uncle would go. I imagine there were ceremonial greetings (waves or honks or something along those lines) when the owners would happen upon one another.
I have a 1984 Buick Regal and seem to get quite a few waves from other GM G-Body car owners.
This too.
I guess Slug Bug wouldn’t work.
When I’m driving my wife’s car ('91 Saab 900), I tend to get waved at every now and then - usually from people in other cars, it’s rare to see another 900 on the road these days. If I’m driving my 9000, I’ll get the occasional wave from other Saab owners, and I’ll do likewise if I see another old Saab on the road.
When my Triumph TR7 was on the road, I used to get waved at occasionally - quite often I’d see people pointing at my car as I drove past. I was never sure if that meant ‘Hey look, a TR7, I haven’t seen one of those in years!’ or ‘Hahaha, look at that old piece of shit!’. I’d just assume the former.
Volkswagen Beetles for sure. My dad owned one in the mid-to-late 60s and everyone waved at one another. (This was in Northern Ireland.)
I’m still amazed that all motorcyclists wave at one another. Why? Some are hard-core biking gang members, and others are weekend yuppies fulfilling a dream. Most are just taking advantage of a cheaper mode of transportation.
Yet, they all do the same underhand peace signal on the way past one another, like they belong to the same cool fraternity of motorcycle riders.
I often make eye contact with other Honda Civic SI owners. Tho the guy the other day in a Fiji Blue one like mine didn’t even look at me nor tried to keep up with me after I V-TECHed away from the red light we were at. Bah…
Muffin waves back.