This is a peperoncino. They're the little Italian peppers that are so good stuffed. Or chopped up in an omelet, or slow roasted with garlic and eggplant. Or used a whole lot of other ways.
What's interesting about them is that sometimes, they're nice and mild and sometimes, they'll burn your tongue off. There's a lot of variability in heat.
This year, I only had one plant that really took off and it turned out nice and mild. The peppers are just starting to redden up now.