Gorgeous weed in the sunflower family (Asteraceae) with a very weedy name: daisy fleabane. It is a member of the very large genus Erigeron, and the species is Erigeron annuus. Each white ray and each little yellow dot is an individual flower, so what looks like one flower is actually hundreds of flowers in a head, which is typical for the sunflower family.
It has been used to repel insects (therefore fleabane!), and also for many other medicinal purposes. This photo is from southern New York State, but it is a common weed in most parts of the United States.