Snakes, I just have always been freaked out. They just seem so evil. A few months ago I almost stepped on a huge black one that was near two others. That ruined my high and was pretty scary.
I'm not afraid of heights but more of collapse, not really so much now, but when I was little I would just freak out thinking that a structure would collapse. Also I don't like being near railings of high places, its not the height that scares me, its the reliance on a rail to keep me from falling that scares me.