sears tower : the ledge
Visitors are now welcome to experience the Sears Tower Ledge -- a glass platform that extends just over four feet out from the building's Skydeck level, so you can look straight down from a height of 1,353 feet. Oy!
These ledges can actually retract into the building, if that makes you feel any safer.
According to the press office, "The inspiration for The Ledge came from hundreds of forehead prints visitors left behind on Skydeck windows every week."