The unusually-shaped London Guarantee building is located at the southwest corner of Michigan Avenue and Wacker Drive, just south of the Chicago River, on the site of historic Fort Dearborn, and looks across the river at the Wrigley Building.
The architectural beauty, unique shape, and historic location make this building a must-see. Judith Paine McBrien writes in the Pocket Guide to Chicago Architecture: Designed by Alfred Alschuler and completed in 1923, "a 22 story building made of Indiana limestone using classical references throughout, from the three story columns

This great photo on the left and more can be found here.
Addendum June 2010:

This is an image of a vintage linen postcard from my collection.
Richard Balsamo