Spider (San Francisco) - family Araneidae, genus probably Araneus diadematus ("European Garden Spider"), female
This is a negative image (positive image is here)
My little friend came back tonight, but this time it's showing me its back! (see its belly 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.