City Hall is the centerpiece of the San Francisco government, and home to the offices of the Mayor as well as the city Supervisors. It has been nicknamed "Willie's Dome" due to its extensive gold refurbishment by Mayor Willie Brown. It was also the site of the infamous shootings of George Moscone and Harvey Milk.
The city hall dome is the largest dome in the Western Hemisphere.
More fun facts about city hall: http://www.ceitronics.com/New_Pages/memorablefacts.html