I think one of my biggest pet peeves has to do with other drivers. I hate when people pull out in front of me, when there is no one behind me, and then they stop quickly and turn. Seriously? It's makes me stabby. And I am guilty of using my horn several times a month.
I also hate when people don't use blinkers at 4 way stops, or really at any time, but mostly 4 way stops.