Funny, this plant and about four other clusters appeared in my garden and I have no idea what it's called. I googled and bing-ed to get acquainted with it but to no avail. Tonight, I finally took out  my "A Garden for All Seasons" book, opened it to browse, when lo and behold, on the very page  was the plant itself.
                                         
                                                           Hello, Viola biflora!!!

Leafy stems are smooth and have a succulent appearance

heart-shaped leaves, edges are slightly serrated



Vivid yellow flowers.
I took this shot at dusk and the flower closed shop for the night.