Although it is no longer the tallest building in the world, and hasn't been for quite some time, the Willis Tower (uh, still Sears Tower to a lot of us) is so tall that if you go to the top of the building, you are able to see four different states.
Yup, if you hang out on the Willis Tower's Skydeck, you can see not only Illinois of course, but you'll also be looking out at Indiana, Michigan and Wisconsin too.
The Sears -- oops I mean Willis Tower -- was built in 1974 and stands at 1,450 feet. It is now listed as the 21st tallest building in the world, with the Burj Khalifa in Dubai being the tallest building at 2,717 feet tall. I recently took a trip to Dubai and I have to admit when I drive into Chicago and look at the Willis Tower, it is considerably shorter than the Burj Khalifa.
But who cares about having the crown for having the tallest building in the world? Chicago is still the greatest city of them all.