People who don't indicate, whether it be turning, changing lanes on a motorway or on a roundabout (I am not psychic!)
People who overtake you and then cut in right in front of you causing you to brake.
Lorries that tailgate you in 50mph sections of the A14
And the one that happened today:- pulled out on the dual carriageway to let someone out (he didn't really leave me much choice) and then he sat parallel with me, I slowed down to drop in behind him so he did too! I had to put my foot down to pass him so I didn't miss my junction.