People who run red lights and stop signs.
People who eat things requiring a utensil while driving.
People who pull out immediately in front of you, do 10 mph and then turn off again 100 feet down the road.
People who don't understand the concept of yield or merge.
People who don't slow down in stowstorms.

And for the first time, I gotta agree with Martin Gibson on something. Female teen and early twenties drivers (especially those with blonde ponytails, for some reason) are extremely aggressive.