Flying Thunder God Kunai

The are  utilized by Minato Namikaze in conjunction with his Space–Time Ninjutsu: the Flying Thunder God Technique. They differ in look from a standard kunai by having three blades instead of one and on the handle (which is thicker than normal) is the "marker" for Minato's teleportation ability; Minato also said that it's a bit heavier than a normal kunai. The prongs on them also makes it more deadly in melee fighting. He seemed to prefer using these special kunai over the standard type.

Minato gave one of these kunai to his pupil Kakashi Hatake, as a celebratory gift for his promotion to jōnin. During the war between Konohagakure and Iwagakure, the ninja from the former threw these kunai all over the battlefield, in order for Minato to teleport all over the place to slay his enemies instantly. He also kept at least one of these blades at his safe house, so he could teleport there quickly when necessary.

Trivia

 * It is implied that touching the kunai activates a Space–Time Ninjutsu, like Minato did to the Nine-Tails' attack with a Space–Time Barrier after using hand-signs.
 * Despite being unnamed in the manga, they were named by Minato in "Naruto Shippūden 4: The Lost Tower".