Citrus sudachi, about 2.5 years old now. Sudachi are little-known outside of Japan, where they are used in a variety of ways. The juice is - along with yuzu and kabosu (two other rare citrus) - a key ingredient in ponzu sauce; the juice is also sometimes used as a salad dressing base in vinaigrette-style dressings. The fruit, which look like small mandarins when ripe, grow in tight bunches like grapefruits, and are EXTREMELY sour - a flavor that is something like a key lime, but a little spicier and more acidic. The juice does contain compounds that - it has been discovered - MAY be of some benefit to diabetics. They belong to the large, and mostly wild (or uncultivated) papeda branch of the citrus family, and are most closely related to kabosu, yuzu, ichang papedas, khasi, and kaffir limes. Like all of those other papeda citrus, the leaves have a spicy-lemony smell when crushed.