I always wave at other rexes. Here in Indiana they aren't too common, and if you do see one it's usually just someone who picked it up for the gas mileage and not a real enthusiast...but I wave anyway. I remember a couple of incidents in my old rex...
Once, I ended up cruising quite a ways on the interstate with a red CRX that I randomly came up on, we were just goofing around, passing each other, hauling ass (I remember doing a little over 100mph side by side, this was on a big interstate with no traffic around) and just generally having fun for a half hour or so until one of us came to our exit. We exchanged a wave and went our separate ways.
Another time in the same car, which was a Y-49, I was on a divided highway and saw another Y-49 headed the other way. It got closer and I realized it looked just like my car, and there was a guy about my age, dark hair like me, wearing a white t-shirt just like me driving. We both looked like we were gonna fall out of the window rubbernecking trying to get a look at each other's cars, knowing that seeing another Barbados Yellow Si in the middle of Indiana didn't happen every day. Awesome CRX moment.