The Oregon anemone is a delicate little flower with many central stamens. Like the sea creature it shares a name with, Anemone oregana does not have petals. Unlike the sea creature it does appear to have petals. But it’s a trick; actually those are sepals that, instead of being green and leafy are colorful, delicate, and–well–basically look like petals. These not-petals can be white, blue, or pink.
The flowers grow in many-stemmed mounds of three-parted leaves. Oregon anemone (also known as blue windflower and western wood anemone) is generally found in moist canyonsides, often under redwoods. According to the Marin Flora, it has been spotted locally including in Mill Valley, Muir Woods, and Lagunitas canyon. I spotted the ones shown here up in Sonoma County. This pretty bloom can be found in the springtime from central California up into Washington.
UPDATE: The flower shown here is actually Anemone grayii, which was recently split off from Anemone oregana. As far as I can tell, the easiest difference between the two to spot is that the peduncle (flower stalk) of the former is lightly to shaggily coated with little hairs, at least towards the top. It also doesn’t come in the pink form. If you really want to dive into the details, you can compare the descriptions of each in the Jepson database. Thanks to Doreen for pointing out the name/taxonomic change!