This thriving little volunteer has me intrigued. Is it a poisonous nightshade?
Uh ... big notched leaves, white tomato-like blossom, clustered fruits.
I would pull it up and add it to the compost heap before the fruit ripens because birds love them and it will spread.
BTW: Black nightshade (Solanum nigrum) ... edible berries and leaves, unlike many of its relatives.
Thank you both for the replies and helping me to learn something new today. I had always believed that all nightshade plants were poisonous except for the fruits of the garden plants tomatoes, eggplants, peppers, and potatoes. But apparently, the black nightshade is used for food by many people all over the world, even the greens! I didn't know this was different than belladonna but it has differences that make it pretty easy to distinguish. http://creationwiki.org/Belladonna
A Forager's Harvest article was really helpful to me, explaining the importance of the black nightshade plant as food. Below is the link in case anybody is interested. I might actually taste a berry once they ripen, if I can get to it before the birds.
Here is a link that might be useful: Forager's Harvest Black Nightshade