A left lane slow passer.
They will go maybe 2-3mph faster than the traffic on the right so technically they are "passing" but at such a slow pace and when there is room to move over they don't and obviously it would appear reckless if I try to pass on the right since usually there would be another vehicle in the right lane up ahead.
And to me the only thing that's about worse than that is the no-signal lane changer.
The other thing that I noticed in general about how people change lanes is that they will almost always start to turn the steering wheel and then put on the signal. It's like they don't pause to check before the start to change lanes. I know some friends who are terrible drivers and when they want to or need to change lanes they almost instinctively start to move the steering wheel and turn on the signal at the same time and of course usually there is a car coming up rapidly and they jerk back. And it's not that they are not paying attention, i think some people are so afraid of driving and traffic that they get so tense and so focused that they actually driver worse. You know the guy/gal who drives with the seat position way upright and they are like leaning forward gripping the steering wheel as if their life depended on it. I guess some people are just not born to be great drivers.