I think it is Galinsoga.  There are two types that I am aware of: Hairy Galinsoga, or Small-Flower Galinsoga.  Hairy Galinsoga is also known as Common Quickweed or Shaggy Soldier (according to my weed ID book).

Scroll down on those links to where it says "more photos" and click. That will bring up better pics.

We had them really thick in our garden when I was growing up.  Our garden had good fertility, and I think this weed likes soils with good fertility.