"...a gardener who knows his flowers and is ignorant of his weeds is like half a coin, a tail without a head." Sara B. Stein
Pretty sure that's http://en.wikipedia.org/wiki/Chenopodium_album.We used to just call it "pigweed" where I grew up in upstate New York, and used to add the young leaves to spring salads. :)
It's lambsquarters, an edible weed!