My sister was in town yesterday and discovered the San Francisco City Guides, a group of over 200 volunteer history buffs who lead over 40 different tours around the city. You just check the schedule of tours online, show up at the designated spot, and some San Francisco enthusiast shows up to walk you around town while telling you stories. It's free, although they do accept donations at the end of the tour.
We saw 2 of them yesterday ("Gold Rush City" and "Nob Hill"), and both were very good, especially "Nob Hill". Our middle-of-the-weekday tours each had about 10 people, so I don't think there's much chance of an awkward, just-you-and-the-tour-guide session.
I strongly recommend the tours, and I'm more than a little embarrassed that my sister had to discover these for me, since I've lived in the Bay Area for almost a decade and she lives in Illinois.