Cyclists that ride on the sidewalk. They're a menace, both to pedestrians and drivers that don't expect them there. I nearly got into an accident with one a few years ago -- I was making a right-hand turn at a stop sign, and there was a building on my right blocking my view, so when I crept out to make my turn, the cyclist going full-speed down the sidewalk (on the wrong side of the road!) nearly went flying over my hood.