...when suddenly I stopped and just gave him the open handed wave - as if to say 'whatever'. I think that actually pissed him off even more than a middle finger because it is almost as if you just couldn't care less how angry the dude is.
Sometimes it's best to do little things like that. Flipping them off gives into their anger & feeds it more. If you smile and/or wave, that takes them by surprise (breaks their state of mind).
About a week ago I'm driving back from the gym. There's a long stretch of highway about 2-3miles long, and it's 4 lanes wide with big wide shoulders. Many people speed down there, and the it's the main feeder to the airport so cabs and limo's are especially popular in this area.
This day I'm coming down the hill that feeds into this straightaway, and I see a Lincoln Town Car blazing down the road. He gets pinned in between cars as he's trying feverishly to get around anyone in anyway he can. After watching him cut across all 4 lanes in numerous variations multiple times- I decided to be an asshole. I'm in a BMW 540, he's in the Lincoln. I'm pretty sure I can mess with him damn good.

So that's what I do. I see every lane change that he's going to make, and I go around the other side to block him in. After doing this for a mile or so, he thinks he's got me around the right side. Nope, I let him blaze up and get pinned between the car in front of him and me on his left, with the shoulder on his right.
By this point he's so pissed at the asshole in the BMW (me!) and really wants to see who is messing with him. He pulls alongside of me, and his face is all pissed looking, and I have the blankest/emotionless face I can try to put on....then I smile at him and wave. I really wish I had his expression on camera!!! He nearly rear ended the car in front of him because he was surprised that I looked at him & made eye contact as well as waving with a smile- he forgot to pay attention to driving!
All in all, I messed with that guy for every inch of highway till I had to merge over and take my exit, and as soon I did so he blazed on never to be seen again...