I’m pretty sure this is garlic mustard (Alliaria petiolata), an invasive species. You can read about it here at Invasives.org for more information. If someone can help me double check my identification, that would be great.
It turns out that this plant kills mycorrhizoidal fungi in the soil that help hardwood trees. It prevents several butterfly species from laying eggs successfully, and it takes over areas already inhabited by rich layers of native plants. It’s a naturalized import from Europe, and right now it is everywhere in the forests around here. It drives me crazy.