Which plant is this?
It looks a lot like Queen Anne's Lace, but yellow and with more slender leaves. I believe it's Sweet Fennel (Foeniculum vulgare). It's in the Apiaceae family, which has a lot of edibles: carrots, parsley, fennel, anise, caraway, celery, chervil, cicely, cilantro, cumin, dill, parsnips and more. Black swallowtail butterflies lay their eggs on this, so often you can find a really fine caterpillar on them; bees like the flowers as well.

Apiaceae has 3,700 species spread across 434 genera; it is the 16th-largest family of flowering plants. It has a few poisonous plants, like water hemlock (white flower) and poison hemlock (also white). Wild parsnip has a very similar yellow flower but it's sap is wildly irritating in the medical sense (it will cause pain and blisters); its leaves are very different. 



If you crush some of the slender leaves between your fingers, it should smell like fennel (a common spice, e.g. in Italian sausage), or if you aren't familiar with that, a bit licorice-like.

 

Sweet fennel (left, up to 6 feet tall)        Wild Parsnip (right, 3-5 feet tall)