South of Market is very quiet, especially at night, except for weekends; take care if travelling in this area, think of taking a taxi if you are leaving a club or bar late.
The Mission District is also known for getting a little unsafer after dark, keep your wits about you and try not to make yourself a target for opportunists.
The Tenderloin was named by the police as they used to get extra for patrolling, it is one of the lower ends of the city, take extra care if you are visiting the area and after dark it is advised to take a taxi or travel in groups.