The Hot TIX booth.
In the Windy City it is the Leaague of Chicago Theatres that runs the Hot Tix booth. With more than 190 member theatres, the Hot Tix booth lists discount tickets on line. You can find a link from their home page.
There are two ticket booth locations, 72 E. Randolph and 163 E. Pearson in the Water Works Visitor Center.
League of Chicago Theatres website