The Second Quest
"The Legend of Zelda" has a second quest, which is essentially a more difficult version of the first quest. It becomes available immediately after beating the game the first time, but there is a trick to unlocking it fully. Start the game as you normally would, but when it asks for you to put in a name, enter "Zelda." When the game begins, you will be playing the second quest.
Keyless Entry Cheat
In "Zelda II: The Adventure of Link," there is a way to get through entire dungeons without using a single key. When you come to a locked door, use the Fairy spell. You'll fly through the keyhole and use some of your magic energy. This can be incredibly helpful if you prefer to speed through the game.
Lens of Truth Cheat
In "The Ocarina of Time," there is an easy way to use the Lens of Truth without depleting your magic energy. When you need to use it, play the "Song of Storms" on your ocarina. When you regain control over Link, activate the Lens of Truth. If you time it correctly, your magic gauge won't flash as it normally does and it will not deplete while you're using this item.
Bottle Cheat
In "The Ocarina of Time," there is a way to replace useless items with bottles. Equip a bottle to one of the C buttons. Use the bottle to catch a bug, fish or fairy. Before the bottle connects with the creature you're trying to catch, pause the game and exchange the empty bottle with an item you no longer need. If done correctly, the bottle will be duplicated and the other item will be gone forever.
Note: Make absolutely sure that you do not need that item any more; there is no way to reverse this process. You could also just make a copy of your game data before attempting this so you have a backup.
The Postman's Game
In "Majora's Mask," the Postman will ask you to play a minigame in which you have to stop the clock after 10 seconds. This can be fairly difficult as the time disappears when the game starts. However, if you equip the Bunny Hood before starting the game, the timer stays onscreen and you can win every time. This is a great way to make money quickly because he pays out 50 rupees per game after you win twice.