This plant spreads by root runners like mint, has clusters of white to lavender flowers that are on a two foot stem.The leaves are going up the flower stem also.
Busybee - it looks like soapwort http://images.google.com/images?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=Saponaria+officinalis&btnG=Google+Search
Could it be Phlox paniculata? The picture is a bit blurry but that is what it looks like.
Busybee, do you have a photo of it a bit further away?
At present I agree with Poppysue (sorry Badseed!) It could be Saponaria officinalis alba plena, the double white soapwort
LOL I don't need to be right Baa. :) I have not grown Saponaria in a few years and had the single blue. It was the two foot stems that made me think Phlox.
Thanks to all of you. I think it looks like the double soapwort in the photos.
Busybee, you have the pretties white flowers growing! I finally started the seeds of that white moonflower bush, and they are coming along just fine. -- Jean