Chamomiles

Rayless chamomile - pineapple weed (Matricaria discoidea or Matricaria matricarioides)

By H. Zell (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

German chamomile (Matricaria recutita or Matricaria chamomilla)

As the flowers age the white petals fold downwards like this:

By Rob Hille (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Roman chamomile (Chamaemelum nobile or Anthemis nobilis)

Distinguishable from German chamomile partly because it tends to be spreading, while Roman chamomile is an upright plant.

By Karelj (Own work) [Public domain], via Wikimedia Commons