This is the Marbled cellar spider (Holocnemus pluchei), one of the most striking spiders I've ever seen in my back yard.
The Marbled Cellar Spider is actually native to the Mediterranean. It was first noticed in North America in Sutter County, California in 1974. This species closely resembles the common long-bodied cellar spider (Pholcus phalangioides), so it may have been here even earlier than that.
This species is more common outdoors than indoors, unlike the long-bodied cellar spider. It builds irregular, dome-shaped nests that are most often communal, being shared by up to 15 individuals.
When feeling threatened, the spider will rapidly vibrate its web so fast that the spider almost seems to disappear, which is probably quite confusing to a predator but a major inconvenience for a human photographer.
This is my usual view of these spiders -- their rear end, as they sit upside-down in their webs.
Although the average size of the female is between 6 to 8mm (males slightly smaller), I have seen a couple of them in my back yard whose bodies were probably 10mm long -- a little scary. They looked like they could beat up the tough kid on my block. But the rest of us have nothing to fear from these household pest-eating arachnids. They are good to have around.