Master Sword

Link's primary weapon in most of The Legend of Zelda games (the Master Sword did not appear in the N64's The Legend of Zelda: Majora's Mask).

In Ocarina of Time, not only is this an important weapon, but Link uses it as a key that lets him travel back and forth through time. The sword is found inside of the Temple of Time. Go up to the altar and play the Song of Time. The three gems Link spent the first part of the game collecting (the Kokiri Emerald, Goron's Ruby, and Zora's Sapphire), will activate and place themselves onto the altar. This will open up a wall leading to the shrine where the Master Sword rests. By pulling the sword out of the stone where it has been placed, Link becomes the Hero of Time.

Unfortunately, this has some nasty side effects. For one, Link is too young to become the Hero of Time (he's roughly 10 years old at this point), so he is sent to the Temple of Light, in the Sacred Realm, to sleep for a number of years until he is able to become the hero as was foretold by the Great Deku Tree. This wouldn't be so bad, except that it also opened up the gateway into the Sacred Realm, so Gannondorf was able to enter and steal the Triforce. However, as Gannondorf was not pure of heart, the Triforce split into it's three parts, and the Triforce of Courage and the Triforce of Wisdom disappeared, leaving Gannondorf "only" with the Triforce of Power. Since our young hero was busy sleeping all this time, Gannondorf pretty much had free reign over Hyrule for quite a few years, and by the time Link wakes up, everything is a real mess (zombies all over the place, ghosts running wild, giant black castle floating in the sky, etc).

Link can return to his younger days by placing the sword back into it's stone in the Temple. This is most useful, as there are many things Link can't do in the later time period, like planting magical beans and using masks. In addition, part of the Spirit Temple requires you to use young Link to get into some tight spots.

The Master Sword deals twice as much damage as the Kokiri Sword. It is also the most powerful weapon Link has that he can use along with a shield - the Megaton Hammer and the Biggoron's Sword both require two hands to use.

In 2002 Nintendo and Capcom released a pair of Zelda games for the Game Boy Color: The Legend of Zelda: Oracle of Seasons and The Legend of Zelda: Oracle of Ages. The Master Sword appeared in each of these games, but in order to earn it was necessary to own both games. Here's how it works: Link begins with the Wooden Sword. Later in the game (either one, they both work the same) the sword can be upgraded to the Noble Sword. Now, after completing the game Link will be given a special password for use in the other game. During the second game Link will meet someone with a special message for Link to take back to the first game. By doing so Link is issued a challenge and completing that challenge awards him with the Master Sword. Link can then take the sword back to the other game with the aid of another password. Like in its earlier incarnation the sword can fire a blast of energy with Link's energy meter is full. Usage of the sword is considered a reward to gamers who have purchased both of the games. It can deal more damage than either the Wooden Sword or the Noble Sword.

During the events of The Legend of Zelda: The Wind Waker Link manages to climb to the top of the Tower of the Gods and complete the tasks set before him. His reward? Access to Hyrule Castle, hidden away deep beneath the sea. Hidden in the castle is the Master Sword itself; the very same one as seen in The Legend of Zelda: Ocarina of Time. However, the sword is now over 100 years old and it's blade has gone dull. By removing the sword from its resting place Link has released the evil Ganon's magical powers, but if Link can restore the sword to its former glory, he just might be able to save the world.

The Master Sword is an important part of Zelda lore. I'd be surprised if we've seen the last of it.