Grappling Hook

The Grappling Hook is an extremely useful item that appears in The Wind Waker and Phantom Hourglass that enables Link to latch on to cantilevered ledges called Grappling Posts, and use them to swing across large distances to otherwise unreachable platforms.

The Wind Waker
The Grappling Hook in The Wind Waker is given to Link as a gift from Medli after defeating the Moblins that had captured her at the summit of Dragon Roost Cavern. A separate function of the Grappling Hook is in the stealing of enemy spoils, i.e. a Joy Pendant, a specific foe carries along with them, and the addition of those small treasures to Link's own Spoils Bag. The Grappling Hook was said to have been used by the Rito people before they obtained wings to navigate the heights and upper levels of the mountain on Dragon Roost Island.

When equipped aboard the King of Red Lions, the Grappling Hook becomes a mechanical crane, which allows Link to pull up Treasure Chests from the seabed. These sunken chests can be found beneath the clustered Light Rings scattered across the Great Sea, or by using Treasure Charts.

Phantom Hourglass
The Grappling Hook reappears in Phantom Hourglass. Here, it can be used like a Hookshot, and also as a walkable tightrope, and as a slingshot, blasting Link across large holes. As a Hookshot, it acts as a kilter of weight, pulling Link towards heavy objects while bringing light objects to him. It is used to defeat the boss, Gleeok. The Salvage Arm replaces the Grappling Hook's crane feature at sea. In this game, it can also redirect arrows, which is useful for shooting certain types of eye switches that close when Link looks at them.

Trivia
This item may have helped pave the way for the Clawshot in Twilight Princess, which has some capacity of both the Grappling Hook and Hookshot.