The flowers of Philadelphus coronarius aureus, or Golden Mock-orange, a cultivar of Philadelphus coronarius. The aureus cultivar has bright golden yellow leaves, and is a popular shrub in gardens.
Below from Wikipedia: http://en.wikipedia.org/wiki/Philadelphus_coronarius
Philadelphus coronarius L. (Sweet Mock-orange, English Dogwood) is a species of deciduous shrubs in the genus Philadelphus, native to Southern Europe. As well as some other species and hybrids in the genus, it is a popular ornamental plant in temperate regions with a large number of named cultivars, valued for its profuse sweetly scented white blossom in early summer.