The Painted Ladies! Ugh - okay the THIRD thing that come to mind when they think of SanFran is the streetcar.
To be honest, I've never actually ridden one; I've never had a need. I take the less glamorous Muni, BART, or Caltrain. However, when I visit my friend, Adam, in Castro I like to get a coffee in the morning and watch the streetcars turn around at Castro and Market. I usually don't stay too, too long because a lot of nudists and homeless people (or worse - a combination) like to park it there ("it" meaning their bare asses and shopping carts)
It's pretty cool because SF uses incorporates "Wheels of the World" so you get a glimpse of all different variations of the streetcar. Check it out.
...and don't even with the pants. I was about to go running.