Probably the most highlighted technique Monkey D. Luffy has in his attacks in the One Piece series is the Gomu Gomu no Hammer. This was one of the earlier moves Luffy had made, showcasing just how creative he could be with his newfound rubber body gifted by the Gomu Gomu no Mi Devil Fruit. Basically, it involves the spinning onto of a stretched arm to catch an enemy and then pound him into the ground, hence the name Hammer.
This technique has been known as different names in various English adaptations of One Piece. The 4Kids and Funimation dubs refer to it as "Gum-Gum Giant Hammer". This localization keeps the hammer theme but places emphasis on its scale of attack.
The Gomu Gomu no Hammer has been used by Luffy in significant fights, especially during his earlier adventures. One of the most memorable occasions where this technique was notably used was when Luffy was fighting Don Krieg. Using the Hammer technique, Luffy was able to hit Krieg straight in the head, showing both its power and accuracy. This forms a good example of how he tends to aim at particular weaknesses or weak points that his opponents might have with his techniques.
Luffy's techniques evolved as his abilities did throughout the series. Gomu Gomu no Hammer remains one of his most significant, and through the series, there have been variations and improvements in the technique as Luffy developed his Gear Second technique. It is a state where he gears up his speed and mobility by pumping blood through his body at an accelerated rate and has given way to the formation of more potent forms of his basic attacks.
One of these versions is the Gomu Gomu no Jet Hammer, which is essentially just a powered version of the original Hammer technique through Gear Second. It has been listed among Luffy's greatest Gear Second techniques but has only been used a few times. Notably, it was seen during the New Fishman Island arc, as it helped prove the fact that this technique can be very useful in high-stakes battles.
Another version of this would be the Gomu Gomu no Gigant Hammer. It would most likely retain elements of Luffy's Gear Third technique to increase the size and power of the attack. However, much like many of Luffy's specialized techniques, the Gigant Hammer has been used very rarely, making it one of his rarer and more situational attacks.
The Gomu Gomu no Hammer and all its variations show the creativity of Luffy in combat and how he could adjust his powers of rubber in many situations. If used not as much as some of his other signature moves, such as the Gomu Gomu no Pistol or Bazooka, the techniques of Hammer remain quite variable in Luffy's way of fighting.
Worth noting is that as the series has gone on and Luffy's opponents have become stronger, some of his older techniques like his basic Hammer have been used less frequently. It shows his growth as a fighter and that the challenges he meets are getting higher, so he needs more advanced techniques and combinations of his abilities.