Purple Deadnettle (Lamium purpureum) 2012-4-4 PL, DE
Purple Deadnettle is in the Mint family and therefore has a square stem. Also shown, stalked leaves (Henbit, with which it can be confused, has no stalks). Purple Deadnettle is listed as an invasive in some parts of the country.
Shown: growth habit, stalked leaves, closeup of flower (about 1/4" long), seed, and square stem. In deep shade the plant may not show a purple top and the flower may appear light pink to white. Picture of seed added 4/4/12.