What is a must-see historical site or building in Chicago?

I recently toured the Driehaus Museum and was absolutely taken by the restoration!!! This was the mansion of Samuel Mayo Nickerson, and the 5 year restoration includes both original pieces from the 19th century home, as well as period pieces from the Driehaus collection, including a stunning display of Tiffany glass. It's an exceptional property, and the guides are phenomenal!

This museum is a recent addition to Chicago's wealth of historic sites. Others will suggest you visit The Rookery, Glessner House, Robie House, the Chicago Cultural Center (with Tiffany Dome), You should consider seeing them, too!