This is a small plant called common starwort (Stellaria graminea). At least I think it is, there are some very similar relatives:
Olympus Stylus 1s, 42mm, ISO160, f5.6, 1/160s
Another modest white flower, of the wild strawberry (Fragaria vesca):
Olympus Stylus 1s, 42mm, ISO200, f5.6, 1/160s
The fruit of the wild strawberry is quite small, but gives an explosion of taste when you eat it; the cultivated strawberries don't even come close.