Kunai

Kunai, along with the shuriken, are a common ninja weapon in Naruto. It is a black dagger designed for thrusting and stabbing, though it can still do some damage if thrown despite not being designed for it, and is about the length of one's hand. Many ninja attack by attaching an exploding tag to the blade. The kunai is usually thrown so that it lodges in an object near the target, and then the explosion from the attached tag destroys said target. Kunai can also be kept on spring-loaded wires in the user's sleeve, as seen in Naruto's fight with Sai when they first met. They can be used for close combat, like a normal knife. Empowered with "chakra flow", kunai thrown into targets can cut through solid wood and even solid stone targets.

The ninja appear to use them so much due to both their sturdy and dependable nature and their versatility.

Types of Kunai
Different types of kunai have been seen in the series:
 * Minato Namikaze used three-bladed kunai that had a special seal applied to them in order to perform his Flying Thunder God Technique.
 * During their battle with Hidan, Izumo and Kotetsu used kunai the size of ordinary blades to attack him.
 * Kabuto, when threatening to kill a hospitalized Sasuke in Part I, held a kunai with a curved blade to Sasuke's neck (seems to be the same type that Rōshi used).
 * During the Chūnin Exams, Team Oboro used double-bladed kunai while facing Team 7.
 * Shiin used kunai with long, thin, red ropes attached to them.
 * Before the first part of the Chūnin Exams, Zaku used a kunai with holes in the middle and no ring at the end.
 * During the first part of the Chūnin Exams Kotetsu Hagane threw a kunai at Sukima, that had fur connected to the ring.
 * Zabuza Momochi was shown using a special kunai that curved inward on one side, making it more suitable for stabbing and thrusting. This type is also shown used by Kirigakure in the anime.
 * In the anime, Naruto finds a special kunai with an extending blade on one side.
 * The Prajñā Group used very long kunais to fight.

Trivia

 * The alternative spelling for "kunai" is written with the characters 苦無, which translate as "no hard feelings." ("bitter", "hardship" and/or "suffering" (苦),  and "negative", "no", "not", "lack" and/or "have no" (無).)