Driving - SUV drivers whose idea of changing lanes is to simple veer into a car. Buses do this too, but at least the driver isn't on a cell phone at the time.
Public Transportation - people who bring ungodly foods on, including fried chicken, seafood platters, hoagies, pizza, etc. I've been guilty of this in the past, but it still drives me crazy.
Movie theatres - a tie between people incessantly flipping their cells open to check the time (note: it's five seconds later than the last time you did it) and the people who bring fried anything into the theatre; french fries, onion rings, and cheese fries DO NOT belong in a movie theatre.
But my overall, general pet peeve is people who behave as if they were raised by autistic cats. I mean the guy who walks a snarling Rottie on a flexi-lead on a city sidewalk, or the woman who observes detachedly as her child slams itself into the legs of hapless passersby.