Mushroom

Mushrooms exist in six Zelda games:A Link to the Past, Link's Awakening, Ocarina of Time, Oracle of Seasons, Majora's Mask, and The Minish Cap.

In A Link to the Past and Link's Awakening, the Magic Mushroom can be respectively found within Lost Woods and Mysterious Forest. In either game, Link can offer it to the witch in exchange for Magic Powder.

In Ocarina of Time, the Mushroom is part of the Adult trading quest. Grog gives an Odd Mushroom to Link in exchange for Cojiro. The mushroom is then to be delivered to the Old Hag in Kakariko. It goes bad after three minutes (or instantly after teleporting), and cannot be delivered in such a condition.

In Majora's Mask, a Magic Mushroom's scent can be detected with the Mask of Scents and stored in an Empty Bottle. One can be found in the Lost Woods, and another can be found growing on someone's boxers in Stock Pot Inn. These can be sold to the Hag Sisters' shop in Southern Swamp for 40 rupees each, and at least one must be delivered before Blue Potions can be purchased there.

The mushroom returns again as a trading sequence item in Oracle of Seasons. Using a Megaphone, Link can wake up the slumbering Talon atop Mt. Cucco. In exchange for waking him, Talon will offer Link a Mushroom, which can in turn be given to Syrup of Sunken City.

In The Minish Cap, the Wake-Up Mushroom can help Link give the Pegasus Boots

Trivia
In Link's Awakening, the Mushroom is one of many Mario references. When Tarin (who bears an uncanny resemblance to Mario himself, and also shares his love of mushrooms) eats one, he turns into a raccoon (although Mario himself required a different power-up item, the Super Leaf, to turn into a raccoon).