City Rent-A-Car is a locally-owned business. They have two locations in San Francisco -- on Folsom across the street from Rainbow Grocery, and on Bush Street near Van Ness.
I'm a member of City Charshare also -- members get a discount at City Rent-a-car. City Carshare doesn't work so well for long-distance or overnight trips, so for those occasions, I rent from City Rent-A-Car.
One caveat is that on weekends they close at 4 pm, and only the Bush St. location is open on Sundays. So if you need an afternoon pickup or dropoff, that's an issue. Other than that, I've always been happy with their cars, their prices, and their friendliness.