Formerly known as the Sears Tower, the Willis Tower was the tallest building in the Western Hemisphere (until New York took the honor). Here is the Skydeck Chicago, which is found on the 103rd floor and provides 360-degree views of Chicago. On a clear day, visitors can see four states and up to 50 miles. If you can stomach it, try The Ledge, which is a glass balcony that extends out and above the streets of Chicago.
Click here to read more about Skydeck Chicago.