Headquarters: The headquarters (palace) is your head of operations. If your headquarters is captured, you will have to place it elsewhere and it is likely that your government will fall in the meantime (or something else bad.)
Church: It's a place of worship and it makes people happy. So when the people are blue, seek help from the man upstairs :)
Barracks: Once you aquire the ability to train better troops from city council, you can build barracks to do so. Barracks repair ground units in one turn and train veteran ground units.
Salary: When the amount of mobsters you have in one area grows to a large number, they often demand a larger piece of the pie, you might as well give it to them in the form of Salaries.
Bar: Many of your mobsters love to go to the bar to discuss the day's events. Building a bar for them can greatly help your local morale. (Seeing as a bunch of disgruntled mobsters does not settle well with the local populace.)
Bribery: If the voices of the people fall on deaf ears, it could also be convenient for you to grease the palms of certain members of city council so that they can hear better.
Sewer System: All large cities have sewer systems, and if your hideout does not have working plumbing, it's not pretty to see a bunch of mobsters going around looking for a toilet in a large population area.
Superhighways: Superhighways represent a large interchange of higway ramps in that station's vicinity and can give a greatly needed boost to commerce in the area.
Gun Shop: When your mobsters need weapons, the Gunshop is usually the best way to supply them with the goods, effectively increasing your capacity for destruction by 50%.