Oxalis corniculata L.
Family: Oxalidaceae
creeping yellow wood-sorrel
Oxalis corniculata image
from USDA Plants website  
Etymology: Oxalis: from the Greek oxus for "sour," referring to the pleasantly sour taste of the leaves and stem
Plants: erect to drooping, perennial, up to 16" tall forb, hairy, creeping, weedy Leaves: 3-parted, alternate
Flowers: yellow, 5-parted, 1/3"-3/4" wide; inflorescence an umbel-like, un-branched cluster of stalked flowers from the leaf axils
Fruits: grayish, 1/2"-1" capsule on a spreading or slightly bent stalk; seeds brownish with white ridges