This IS a great question. Litterbugs!!! I still can't believe that there are people who will simply throw a soda can, a fast-food container or a water bottle out of the car window. And yet it happens so much that when it comes time to clean the shoulder of a highway, you see bag upon bag upon bag of trash that's been collected, waiting to be picked up. It's so easy to throw stuff away these days, because everywhere you stop -- gas stations, convenience stores, grocery stores, etc -- have trash cans outside their doors. It's a no-brainer to just keep the trash in the car until you stop somewhere, and get rid of it there instead of dirtying the landscape with it. But arrogant people still insist on tossing it out the windows without thought.
I think my other pet peeve is people who believe that they are busier than anyone else. It goes along with Alison's peeve about inconsiderate people in some ways. These folks are the ones who can't merge regularly when traffic is stalled (even going so far as to drive on the shoulder), or who sigh and complain when a line is long, or who rush ahead of you to get into the revolving door first, and so on. Everyone is busy, and we're all trying to get somewhere, do something, or find something, and I hate it when one person believes their drive, search or accomplishment outweighs everyone else's.