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 people you have in one area grows to a large number, they often demand compensation for their work, so you might as well give it to them in the form of Salaries.
Bar: If your gangsters are down, you could always make them more down, but feel up, with a Bar!
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.
Nuclear Detector: If the Mobsters should happen to receive nuclear terrorists, it may become necessary for you to build Nuclear Detectors to prevent the Mobsters from destroying the town.
Sewer System: C'mon, even the criminals have to go ever now and then.
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 gangsters need weapons, the Gunshop is usually the best way to supply them with the goods, effectively increasing your capacity for destruction by 50%.
Aphrodesiacs: When you need more people fast, Aphrodesiacs are the best way to get them motivated.
Barracade: The barracade can keep out unwanted visitors by giving your defending units a boost in defensive values. Besides, who would ever want to come through a few feet of razor wire fence?
Red Light District: When your people are really down and out, a Red Light District can help to keep their minds off of the subject.