Prairie Loosestrife. The only ones I've found were growing in quite wet areas; specifically fens.
This photo illustrates this plant's alternate common name: Four-flowered Loosestrife; and its Latin name: L. quadriflora ("four-flowered").
Note the typical, yellow, nodding loosestrife-like flowers.
Note how the flowers stand out from the stem on thin stalks. And the flowers are nodding (point somewhat downward).
The distinctive narrow stiff leaves. These, combined with the yellow, loosestrife-like flowers, ID this plant.
Closeup view of a flower. Note the typical pointed loosestrife petals, pointed at the ends.
