The Bridge on the top of Nevada Falls (594 feet, I think). The Merced River was flowing like a dam had burst, and sounded like a freight train. This bridge stood right on the edge, and we could watch all of the water racing towards us, underneath our feet, and then over the edge. Viewing this bridge and waterfall from the other side of the valley gives an amazing sense of scale. I'll post pictures of that elsewhere.
In fact, view here http://www.pbase.com/ardave/image/57544748/original to see the waterfall from further away. Look at the upper waterfall to the right of Half Dome.