Via About.com I have yet another reason to visit San Francisco: the Vampire tour of San Francisco
Explore the Gothic side of Nob Hill. Meet Mina Harker, Vampress who was made by Count Dracula in London in 1897, and banished by him to the United States. Mina has resided on and under the streets of San Francisco for over 100 years. Hear her story, and learn how Vampires have played an important part in the shaping of San Francisco as we know it today.
This sounds like a fun way to learn the city's dark history. I'd also want to take the Dashiell Hammett tour. I'm going to have to catch me an airplane and fly out there.