St. John's wort (Hypericum perforatum) is an invasive plant with medicinal properties. In fact, "touch-and-heal" is one of its other common names. Cathy Wong, About.com Guide to Alternative Medicine, calls it "one of the most commonly used herbs in the United States." As Cathy alludes to, the common name derives from the fact that, in its native Europe, the plant was observed to have bloomed at about the time of the birthday of St. John the Baptist.
This saint's birthday (June 24) falls right around Midsummer's Eve. That was a very special occasion in pagan times, marked, for example, by bonfires, upon which highly prized herbs would have been tossed, according to Richard Mabey. In his book, Weeds, Mabey writes (p. 75), "During the Middle Ages, the Midsummer fires were appropriated by the Christian Church, and said to be lit in honor of St John.... And the distinction of receiving the saint's name fell on the most magical of all the plants in the fire-mix," namely, St John's wort.
Also spelled "St. Johnswort," this invasive plant (invasive in North America, that is) is used to treat depression. It has naturalized in parts of the U.S. Some people find its bright yellow flowers pretty.
Do not confuse the perennial flower discussed above with the types of St. John's wort that come in shrub form and are grown for their colorful berries. These types belong to the same genus, Hypericum but are a different species.