What grinds my gears:
Co-workers who spend more time gossiping and gabbing than doing any actual work. I work myself to the point of exhaustion and mental instability, and while I don't expect other people to have my near-insane work ethic, I get seriously pissed off at anyone who whines about how hard their job is when they only do any work for about 3 or 4 hours out of an 8 hour workday. I keep my cool, and manage to get through my day without slapping any of these people, but once I get home, I rage to anyone who'll listen about my lazy, worthless co-workers.
People who just won't shut up about their kids. Actually, this one is gaining on the work thing lately. Whoop-de-doo, you can procreate. Big deal. I like kids as much as the next person, but good god, do I really have to sit and listen to someone drone on for an hour about how little Billy made his first poopy? And it's not like I'd even have anything to add to the conversation if it weren't totally one-sided, unless I felt the need to throw in talk of my cats' exciting lives of sleeping and being able to use a litter box.