I live in southern Illinois, and earlier today I noticed this small red fruit in my yard. It looks kind of like a strawberry, but it's the size of a small raspberry (about 1/4 inches in diameter).
Does anyone have any idea what it is?
That is Mock Strawberry, Duchesnea indica. The fruits are edible, but rather tasteless. This is a common weed in much of the United States. Look for small, strawberry-like plants in your lawn, with long stolons connecting the individual crowns. These plants have five petaled yellow flowers. See comparison photos: