Gomu Gomu no Mi Devil Fruit Monkey D Luffy

Luffy has gained extraordinary stretching powers from eating the Gomu Gomu no Mi (lit. Rubber Rubber Fruit, Gum-Gum Fruit in the English dub), which is one of the rare and legendary Devil Fruits. 

Gomu Gomu no Mi Devil Fruit 

After consuming the Gomu Gomu no Mi, Luffy's body became like rubber, giving him the ability to stretch long distances. Having trained hard for ten years, Luffy is able to supplement his stretching powers with immense strength, agility, and creativity in combat. His fighting style is largely improvised, and heavily reliant on his Devil Fruit. It should be noted, Luffy cannot automatically stretch his limbs (like superheroes with stretching powers, i.e: Mr. Fantastic or Plastic Man) since his body acts more like a rubber band, thus has to throw some force into it in order for his limbs to stretch out, though it seems that he has managed to be able to do it automatically after the time-skip. However, he can hold his limbs outstretched for a long time, as seen when he reaches his arm out about half a mile to retrieve Usopp from falling through the Sea Clouds. Luffy uses his stretching power in ingenious ways. 

Gomu Gomu no Mi Devil Fruit 

He is able to use the malleable stretchiness of his limbs to gain tremendous velocity and hit enemies with devastating impact. His rubber body is also extremely durable and virtually immune to blunt objects such as bullets (in One Piece, bullets are still round pellets, except for Don Krieg's), punches and electricity, bouncing off his body harmlessly or canceling out attacks. It's even become a running gag that people find out the hard way Luffy is made of rubber when they shoot at him, only to have the bullets bounce right back. Another running joke is that whenever Luffy's body is in peril that would kill a normal human, he says that he's made of rubber, so it's okay, or acts like something was a small problem despite the person trying to kill him, such as being shot in the heart. However, he is still normally susceptible to sharp objects such as swords and knives as well as nonphysical attacks like heat, cold, dehydration, and poison (though after the timeskip, Luffy has shown a strong immunity to poison due to his fight against Magellan in Impel Down). Not only can he use his ability to increase the range and strength of his attacks, but he can also use it to restrain and even control foes like puppets.