Here in southern Maine, Black Elderberry (Sambucus nigra) is blooming, which makes now a great time to notice this widespread shrub. Black Elderberry typically grows in moist soils, so look for the bright white flower clusters lighting roadside gullies and the edges of small streams. Many of these clusters will produce forage-able purple/black fruits by September, but the flowers themselves can also be tinctured or dried for tea.
In Holistic Herbal (1990), David Hoffman writes that preparations of Black Elderberry flowers are "ideal for the treatment of colds and influenza" (p. 197). Consult his book or another trusted herbal reference for more details on making medicine with this plant.
Field marks for Black Elderberry include large, flat-topped flower clusters with hundreds of tiny 5-petaled flowers*, pinnately-compound leaves arranged oppositely along stems, toothed leaflets, and young stems and older bark with obvious freckles (technically lenticels). These features are shown in the following images. (To view them in full-size, click here.)
*Warning: Do not confuse Black Elderberry flowers with those of Spotted Water-hemlock.
Note: See the comments section for a discussion on Elderberry toxicity and the edibility of Red Elderberry flowers and fruits.