What is this berry?
Great picture and timely question. The plant you are holding appears to be Bittersweet Nightshade and is quite common across the US. The plant is in the same family as tomatoes (Nightshade) and you will notice the berries have a very tomato-like appearance. Your picture didn't show any of the flowers, but the plant, which is usually vine-like, has rather attractive bluish flowers with yellow centers which are somewhat similar to those of the eggplant which is also in the Nightshade family.
Now the bad part: the plant is a noxious weed and is mildly poisonous. The biggest problem is that its appearance is enticing to children, livestock and pets and therefore should be removed when possible. Here is a link to a fact sheet from the Oregon State University Extension on Bittersweet Nightshade that will provide additional information. http://horticulture.oregonstate.edu/content/bittersweet-nightshade
Thank you for your question and enjoy the summer as much as possible!