People who have phone conversations while standing at the urinal. WTH?
Also: people who mozy on down the street talking on their cell phones, start swerving into your lane until you alert them to their incredibly dangerous actions with the loud report of your horn... and somehow, you're the one who should go <middle finger> and <expletive gesture> yourself. (Ah, what I wouldn't do for a directional EMP attached to my truck...)
Then there are those people who block an entire lane that's COMPLETELY OPEN for MILES in front of them so that nobody can get by. This goes for idiots on the road AND idiot walking down isles/hallways/stairs/etc blocking the entire walkway.
Then there're those people who sit in a hallway waiting for classes sitting on the ground with their legs fully extended so there's a 1-foot (~1/3 meter) gap in the middle that's safe for walking - and again, your gestures of dissatisfaction are met with disgusted 'I'm the center of the universe' expressions.
In general, I guess there are three types of people:
Those who are too stupid to know they're stupid;
Those who are stupid enough to believe they're smart enough to be able to run all the stupid people's lives better than they can; and
Those who aren't stupid and don't give a crap about running other people's life and resent people trying to run theirs.
Many people think they fit in category three, but really fit in category two. Many people don't think, which therefore places them in category one.
The first two groups piss me off, but only one causes true resentment.