Could be a stitchwort or a chickweed (Stellaria sp. or Cerastium sp.).
I'd say lesser stitchwort (Stellaria graminea). Most Cerastiums have five styles, not three. The deeply bifid petals distinguish S. graminea from S. holostea, but it could be wood stichwort (S. nemorum, or great chickweed (S. neglecta) - the foliage would distinguish these.
I think you're right Ernest, Stellaria graminea... see below for comparison:
thanks everyone for the ID. i think this flower is really aesthetically pleasing, and i am glad to know what it is.
Apparently the five petals with a textured outer edge is sort of the theme for the carnation family, Caryophyllales. Maybe I'll be able to spot some now.