Bearded Iris (Iris germanica).
Bearded iris is a widely cultivated plant and many cultivars exist. Flowers can range from white to yellow or red to violet. The way to differentiate this species from others is by the well developed aerial stems, the bearded flowers, and the foliaceous bracts that subtended the flowers. More pictures of Iris flower in Iris Gallery and in the Iris page.
If you want more information about this flower, just search the plant's Latin name on Google: