Probably Acacia baileyana, this is one of the first blooms of the New Year. My neighbor called this mimosa. I love that name. I moved to California from the midwest, after a short stint in Boston, and this was a discovery for me. The bright, cheerful, fuzzy-looking balls were kind of magical. The foliage is a feathery, finely cut blue-grey. The blossoms are very fragrant and when we first moved here I would cut big bunches and bring it into the house. It seeds out and some find this plant weedy. It might be the California answer to Forsythia, which is not widely grown.