In the end of the game, if you have gotten all the "happy masks" and played with all the little masked boys at the big tree inside the moon, thereby giving all the masks away, you will be given the obscenely powerful Fierce Deity's Mask.

This mask makes Link look similar to adult Link from Ocarina of Time. It also gives him the Helix Sword, which makes Biggoron's Sword look like a plastic toy knife. With this mask on, Link can shoot mega-powered disks of energy that can cut through the defenses of any enemy.

You can only wear this mask for Boss Fights, and the only boss it really doesn't work good against is Twinmold, the boss of Stone Tower Temple.

Actually, you can wear this mask outside of a dungeon, but there are many limitations.

It must be on day three. Go to the place in Ikana Canyon where Kafei hides. Wear the bunny hood and run against the crack in the door until you slip through it. LEAVE THE BUNNY HOOD ON! Do the little mini quest with the blocks until you gain control of Kafei. While controlling Kafei, go into the subscreens and switch the bunny hood with the Fierce Deity's Mask (use the same button). When you regain control of Link, he will be forced to wear the Fierce Deity's Mask.

Finish the quest as per normal, and when you get out, fall down the waterfall to get back into Termina's mainland. Now you can run around as the Fierce Deity, but only for as as much time as is left, and if you go back into Clock Town, the guards will not let you back out.