Lobelia cardinalis. Seen in the woods in Wells, Maine, USA.
From Wikipedia: "It is a perennial herbaceous plant that grows up to 1.2 m tall and is found in wet places, streambanks, and swamps. The leaves are up to 20 cm long and 5 cm broad, lanceolate to oval, with a toothed margin. The flowers are usually vibrant red, deeply five-lobed, up to 4 cm across; they are produced in an erect raceme up to 70 cm tall during the summer to fall. Forms with white (f. alba) and pink (f. rosea) flowers are also known.
Lobelia cardinalis is related to two other Lobelia species in to the Eastern United States, Lobelia inflata (Indian tobacco) and Lobelia siphilitica (great lobelia); all display the characteristic “lip” petal near the opening of the flower and the “milky” liquid the plant excretes. L. siphilitica has blue flowers and is pollinated by bees, whereas L. cardinalis is red and is pollinated by the ruby-throated hummingbird (Archilochus colubris)."