A beautiful pile of Banana Chilies on sale in a Thai fresh market.
Banana chilies are called prik yuak in Thai, and the are one of my favorites. They are large yellowish-green peppers, somewhere between the spicy hot chillies and the mild bell pepper. The taste actually puts me in mind of what must be the chilli's ancestor, the jalapeño peppers used in Mexican cuisine. The taste is pungent rather than hot. This makes it an excellent ingredient not only in light stir-fries, but also salads.
Chilies contain essential oils that, taken in moderation, help to stimulate blood circulation and can help prevent heart disease or cancer. If you can't find banana chillies, then look another variety of medium sized chili pepper.