aka spotted touch-me-not, orange touch-me-not, orange jewelweed; Impatiens capensis; Balsaminaceae (touch-me-not family)
This easily-found annual plant grows 2-5′ tall, with flowers borne in small clusters in the upper leaf axils. It likes the wet soils along stream banks, and can be found over most of the US and Canada except the desert southwest, Montana, and Wyoming. In this area, you can also find the less common pale touch-me-not, I. pallida, along the C&O canal in DC (where I saw it last year).
The touch-me-nots are closely related to the very popular bedding plant called impatiens that you find just about any place that sells plants. This native species attracts ruby-throated hummingbirds, and is an especially important food source for bumblebees. Supposedly the sap has anti-fungal properties, and can relieve itching from poison ivy as well.
Supposedly the common name “touch-me-not” comes from the fact that the ripe seed pods explode and fling seeds everywhere at the slightest touch.