Tessia (classmate of mine) and I went up to the sky deck one day while in Chicago, the observation deck at the top of the Willis Tower (previously Sears Tower). Crazy fun!
This was a glass ledge that hung off the side of the building, with a glass floor cantilevering 1,300 feet in the air. Nutso! I didn't think it would be so nerve-racking to stand on, but turns out it did make a me a little uneasy.