Most probably Tripleurospermum inodorum (scentless mayweed / false mayweed / false chamomile). It is a noxious weed, introduced in North America. T. perforatum is a synonym. Earlier, the species also has been treated under the genera Anthemis and Matricaria. Of course, it is a herb and a weed (and a plant) at the same time! Cheers, Harri Harmaja http://www.fmnh.helsinki.fi/users/harmaja/about_myself.htm