Crash pretty much has it covered.
Additions:
When people doing 10 mph UNDER the speed limit occupy all lanes on the freeway and won't move out of the way (For those of you in Houston, I am refering the Westpark Tollway)
When that sweet parking spot in front of you is open and you can pull out, instead of backing out. Then BAM! someone pulls in.
People that sit at lights revving their motors.
People coming onto the freeway assuming they are the Gods of the road and you
must yield to them.
People that ride their F*@&ing brakes!!