Spider (San Francisco) - family Araneidae, genus probably Araneus diadematus ("European Garden Spider"), female
This is a negative image (positive image is here)
This Spider created an almost perfect orb-web just outsite one of my windows, probably catching insects attracted by the lights inside at night.
Size about 1-inch including the legs.
Remember, spiders are good for people. They eat mosquitos and other disease-carring insects and they almost never bite people. They are nice and friendly, they just have a bad reputation because they look spooky with their 8 legs and their 8 eyes :)
Don't kill spiders! If you find one in your house, leave it alone or gently take it outside. Remember, spiders are your friends, they catch and eat mosquitos.