Oh, this is a cute one.  I first discovered this tiny native violet growing in the pot with a plant I had purchased.  I liked it, so I plucked it out and potted it so I could keep an eye on it.  I forgot about it and lost it, but a couple of years later, I found colonies of it growing in the grass at my greenhouse.  I think it had been there all along because it is a native of just about the entire country.  If you're not paying attention in the spring, you just might miss it like I did.  The small white blooms are only about 1/4 inch wide with central violet striations and reflexed top petals.  I warn you that, while it is adorable, it forms colonies all over the place by seed and rhizomes.  I don't mind it in the grass because it is prettier than grass, but if you prefer grass, be careful where you plant it.  Zones: just about everywhere.