Most of the time I let it slide, but if it's some prick who drives like a Formula 1 racer, I just slow down to piss them off more.

As far as cutting me off, if it almost made me crash, I'll prolly just pull up next to them and give them a piece of a my mind.

But for the most part, assholes that are always in a hurry will get their come up-in's soon enough. And probably a lot worse.