Oohh I have to get in on this:
#1: I HATE pricks who pass everyone in a construction zone that is narrowing down to 1 or two lanes as if they have a right to cut in front of everyone else!! I cut in front of the stupid jackasses where possible and make them wait like they should!
My dad did that once to a guy in a nice shiny black Eldorado ... we had a beat up minivan, so my Dad was like "cmon, I'd love to get some of that nice black paint on my bumper buddy!" The guy was forced to slam on the brakes and sit while all the cars behind us passed him.

#2: People who tailgate and swerve around me and other drivers in heavy traffic that's moving slowly. CHILL OUT! I hate being stuck behind slow drivers too, but if you can't do anything about it, then just RELAX!

#3: People who pull out in front of me going 25 when I am doing 50! DO YOU WANT TO GET KILLED??

I usually can stop in time, but there have been several times where I had to swerve, and was very glad that there was no one next to me!
#4: Geriatric drivers who drive 10-15 mph under the speed limit. Maybe they don't realize that the cops will pull you over if you are going too slow!
#5: People who drift back and forth across several lanes on the freeway. If they are coming into my lane I like to pass them REAL CLOSE and then toot my horn and watch them panic and swerve back into their lane. Keep your vehicle under control!

Whew I think thats it for now ...