In 2009, the Sears Tower was renamed to the Willis Tower. The Willis Group acquired naming rights as a result of it's majority lease of the building's space. The largest tenant in Willis Tower is United Airlines.
What is most interesting is how this iconic building was renamed with so little fanfare. When completed in 1973, the Willis Tower was the tallest building in the world - a title it held until 1998.
As such, the former Sears Tower was a celebrated piece of American corporate industrialism and stood for progress, ambition, and the excellence of the American ideal.
The name - Willis Tower - just doesn't seem to command the same type of aura as that of the Sears Tower.