Everything. I find reasons to get pissed off at everything. I'll list the ones that enrage me on a daily basis.
Bad drivers in general, covers anything from tailgating, lack of drivers etiquette, speed or lack thereof, etc.
People who think they're entitled to things.
Stupid people in games that run off to do their own thing like they're a General. Sure, General McDumbfuck.
Children that aren't mine. They're mouthy, ignorant and deserve to get beat, which is why I beat mine when they get lippy. They know better.
Women. They have logic that makes absolutely no sense, except to themselves, and other vaginas.
Bandwagoners/Fanboys. GTFO, you don't have any clue.
My family. They drive me nuts.
Everyone.
That completes the short list.