Milkweed

Common Milkweed (Asclepias syriaca) doesn't have the show-stopping flowers of the Butterfly Milkweed but the pale pink flowers are attractive enough. They're particularly attractive to butterflies, most famously the Monarch.

Milkweed is named for its thick white sap, which protects the plant from many herbivores since it contains bitter cardiac glycosides that are toxic in large doses. Certain insects, like the Monarch Butterfly, have adapted to tolerate this poison and, in fact, are able to incorporate it as their own chemical defense against predators.

The USDA has an informative pdf on the Common Milkweed.