Other than flight, the highest I've been is near the top of the Empire State Building, but I wasn't really afraid to look down. I was still inside the building.

The highest I've been while actually in peril of potentially falling was probably on top of a rickety set of pump-jacks while installing fascia and soffit board around the eaves of a roof. I worked on a framing crew for a summer during college. For whatever reason, we had the walk board jacked up high enough that I could wack my head on the roof eave - I think we must have been working on the cornice as it wrapped the gable end. At one point I was knelt below the eave and forgot how close I was to it, and tried to stand up and smacked the crap out of my head. I came fairly close to falling off, but thankfully dropped back to my knee and regained my balance.

Another time, later in college, I was working for the head of the VT building construction department during my last summer before graduating. We were building some precast concrete condos in Christiansburg, and for whatever reason, no one ever thought it would be a good idea to install fall protection around the openings for the stairs. I was working on the third floor of the building installing anchor bolts in the top of the partition walls between two condos, right next to the stair opening for the unit I was in. I was working on a straight ladder that was leaned against the partition wall, with no one to foot the ladder for me (all practices generally frowned upon by OSHA) when the bottom of the ladder started to kick out away from the wall and the top of the ladder started to slide down the face of the wall. All I could do was hang on for dear life and try not to fall into the stair opening. I landed with my arms pinned between the floor slab and the ladder, basically looking over the edge of the stair opening as I watched the spud wrench I had been holding fall ~30 feet to the garage floor two stories below. That was probably the closest I've come to getting seriously injured from a fall.