A good, general website for tourists to begin their travel research is "Only in San Francisco".
There are many sightseeing services in San Francisco. The Busy San Francisco Tours website lists all kinds of tours. One of the more popular bus tours allows you to get on and off the bus and your ticket is good for 48 hours.
If you prefer a walking tour, there are many specialized walking tours (architectural interest, comedy, eating in chinatown, vampires, chocolate, etc.), check out the About.com website.