We have been watching this weed grow in the back yard all summer. It has grown to an amazing height (considering that I am about 6'4''). Recently my lovely and talented girlfriend Sara with no 'h' became concerned that it might be Giant Hogweed so she did a little detective work.

With the help of Glenn Nice she was able to determine that it is in fact Tall Blue Lettuce (Lactuca biennis). It is useless, but harmless. More of Sara's photos of the plant here.