Tailgaters
People who cut you off and then drive slowly
People who run stop signs and red lights
People who turn without signaling, or people who signal and then don't turn or change lanes
People who stop FOREVER at stop signs, and yield to people they don't have to yield to
People who leave their high beams on despite the fact that there is an oncoming vehicle
People who sit at an intersection trying to decide if they're clear to make a turn, then when they're no longer clear (i.e. you as the oncoming driver are too close to them), THEN they decide to turn
People who drive slowly, wait for you to try to pass them in a passing zone, then while you're on the wrong side of the road, decide they want to race
Slow drivers in general
Just today, I was driving to work and an oncoming driver was flashing his high beams, so everyone figured there was a speed trap or some other hazard ahead and slowed to the speed limit (30). (BTW, there wasn't.) The limit goes up to 45 about 1/2 mile further down the road. Of course, I'm stuck behind the idiot who misses the new speed limit sign and continues to go 30.