Deku Nut

Deku Nuts are items from Ocarina of Time, Majora's Mask and Twilight Princess.

Location
Deku Nuts are the dried buds of Deku Babas, as opposed to Deku Sticks, which are the stalks. Deku Nuts are odd in that they erupt into a blinding flash on impact with a surface. Although defeating Deku Babas is the most common source of these items, they can be found as well in grass and jars; they're usually for sale at low prices in shops as well.

Uses
Throughout Ocarina of Time and Majora's Mask, Link can use the Deku Nut to temporarily stun enemies (which is indicated by a blue skin that filters over the enemy).When the Nut collides with the ground, it let's off a large flash which freezes the enemy in place.

Historically, the Deku Nuts were famously used by the Sheikah to quickly leave an area unseen--one would literally vanish after throwing one. However most of the time, the only effect of a thrown Deku Nut is that it stuns anything around the flash giving them a fading blue hue until the paralyzing effect wears off. Oddly enough, powerful figures such as Ganon may be injured by Deku Nuts along with very weak creatures such as Shabom.

Deku Nuts are the weapon of choice of Deku Scrubs, which spit them from their mouths. Surprisingly, the nuts utilized by Scrubs do not explode on impact, and instead simply deal damage and crumble; however, that being said, Deku Link is able to drop Deku Nuts while flying using a Deku Flower which have the same effect as throwing them. Besides being used for battle, Deku Nuts also hold aesthetic value for the Deku Royalty in particular, as the Deku King's scepter is adorned with a large Deku Nut on top.

Deku Nuts in Twilight Princess cannot be used as items, but can be picked up from deceased Deku Babas and thrown like jars or rocks. They will not explode or produce a flash, but instead will yield Deku Seeds which can be used with the Slingshot.

Super Smash Bros. Brawl

 * See SmashWiki's article on Deku Nuts for more information.

Deku Nuts appear as items in Super Smash Bros. Brawl. When thrown, the Deku Nut explodes when it hits the ground and stuns any nearby opponents, causing approximately 18% worth of damage. They also appear as a trophy and as a sticker.

Trivia

 * The Great Deku Tree's seeds in The Wind Waker look like giant Deku Nuts.
 * The way the Sheikahs use the Deku Nuts is an adaption of the metsubushi techniques used by real-life ninjas.
 * When Impa and Sheik use a Deku Nut after talking to Link, a very small period of time passes before the impact's light vanishes and the Sheikah dissapear, apparently implying that they go away instantly. It's possible, however, that Link stays stunned for longer, and only recovers consciousness after the item's effect fades away.
 * With the exception of the final blow with the Master Sword, Ganon can be killed using only Deku Nuts on his tail. While more are required than Link can carry at once (even with upgrades) several of the pieces of rubble on the battlefield contain Deku Nuts once Ganon smashes them open.
 * Deku Seeds in Skyward Sword are incredibly similar to Deku Nuts in that they come from Deku Babas and forest plants, produce a flash on impact and stun enemies that come close to the flash. In context, it is possible that without full maturity, the seed is a weaker version of the nut.
 * During the Queen Gohma battle in the original Beta release of Ocarina of Time the game would freeze and need to be hard reset if link attempted to stun Gohma by using a Deku Nut.