The Sky Cannon is a special Cannon from Twilight Princess that allows Link to reach the City in the Sky. It was originally made by the Oocca, but over time had been forgotten and lost.
300 Rupees to repair
Launches Link to the City in the Sky
Retrieving the Cannon
Later in the game, Shad discovers an Owl Statue blocking a doorway in Renado's basement in Kakariko Village. When Link obtains the Dominion Rod he can, with the help of Shad, move the Owl Statue blocking the entrance. This reveals the broken sky cannon what Zant broke into, which Midna can warp to Lake Hylia once Shad is out of the way.
There, if Link shows Fyer the cannon, he will repair it after recognizing it as an old artifact. Once the cannon is back in its original state, Link can grab hold of the back of the cannon with his Clawshot, which then launches him all the way to the City in the Sky.