A countersink with included drill bit is an essential tool accessory. Ideal for installing fasteners at the ends of boards or where they might split, the countersink both pre-drills a pilot hole for the screw shank and bores out a small pocket, or dimple, that accepts the screw head, allowing it to seat just below the surface of the work. The 1/4 inch hex drive makes it awesome to use with a cordless impact driver. Absolutely indispensable tool.
