The so-called shinobigatana (literally, "ninja sword") is said to have been the weapon of choice of Japan's famous ninja spies. Unlike the tachi or katana style swords used by the samurai, which had curved blades to maximize cutting power, the ninja sword was said to have a perfectly straight blade. The reasoning went that the ninja did not have time to engage in a hack and slash melee. Rather, one strike was usually all they would get, and it had better be a good one. Thus the straight blade, which was said to have been better able to penetrate through a chink in an opponent's armor and kill with a single stab.
The other unique feature of the ninja sword was its utility as a climbing aid. The scabbard of the sword had a pointed tip, rather than the traditional rounded one, and therefore the sword could be jammed into the earth, hilt side up, so that the ninja could use the sword's unusually large and uniquely square hand guard to give himself a lift up onto a wall or other obstacle. Once the ninja had clambered to the top, he could then retrieve the sword, pulling it up by the long cord attached to the scabbard for that very purpose.
Nowadays this legendary ninja sword, with its straight blade, square guard, and pointed scabbard, is absolutely ubiquitous in souvenir shops, internet sites, movies, and where ever else ninja make their latter-day money-generating presence felt. In reality, the "ninja sword" was an invention of Edo Era ninja enthusiasts, long after centuries of peace had put most real ninja out of work. There is a reason why real Japanese swords are curved, and in actual combat the ninja sword is a grossly impractical weapon, prone to having its blade snapped off. For real ninja, stealth was of the essence, and a full-sized sword would have been a bit conspicuous. The true weapon of choice for the ninja was the kama, the traditional sickle of the Japanese peasant. Small and inconspicuous, the kama came with a ready-made excuse if discovered - "I am just a simple farmer, sir."