The Legend of Zelda: Majora's Mask/Masks

Masks will be listed by type, and in the order in which you're able to obtain them.

Transformation Masks
These are masks which let you transform into another type of creature, gaining special powers which usually include a special type of instrument, mode of transportation, and magical ability.

Deku Mask
This mask lets you play the bagpipes when you use your Ocarina, lets you skip 5 times on top of water, use deku flowers to jump from one location to another, lets you shoot bubbles with the B Button, and Deku Link cannot touch fire.
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Obtained after completing the first three-day quest and playing the Ocarina of Time on the roof.
 * How to Get

Goron Mask
This mask lets you play the drums when you use your Ocarina, let's you survive being in lava, and allows you to turn yourself into invulnerable spiky ball for easy locomotion. The Goron can not go under water.
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Obtained after completing the quest with Darunia in the Goron Village, after beating the first dungeon.
 * How to Get

Zora Mask
This mask lets you play a guitar when you use your Ocarina, allows you to swim and dive under water, use your fins as a boomerang, and surround yourself with an electric field while underwater.
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Obtained after playing the song of healing for Mikai at Great Bay, after beating the second dungeon.
 * How to Get

Giant's Mask
Transforms you into a giant during the Twinmold boss fight, although it drains your magic rapidly while you use it.
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Obtained in the fourth and final dungeon. You can use some Chateau Romani to have an infinite magic meter while you're in this form, or break the pillars for magic jars.
 * How to Get

Fierce Deity's Mask
Turns you into an incredibly powerful fighter which can only be used during boss fights. You can also fire lasers from your sword by targeting and swinging your sword even if you're not in range. Very fun mask once the game is beaten.
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Obtained by finding every mask in the game and trading them to the kids on the moon after beating all four dungeons.
 * How to Get

Masks with Special Powers
These masks are ones which give you special abilities that may come in handy during your quest.

Great Fairy's Mask
This mask will draw stray fairies to your location and will cause your hair to glow when there are any in the room you're currently in. Obtained by rescuing the Stray Fairy in Clock Town and visiting the great Fairy when you're in Link's form. Can be done after the first 3 day quest.
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 * How To Get

Blast Mask
This mask will allow you to explode at will, coming in very handy if you run out of bombs. Doing so will cause damage to Link, however, if you raise your shield before detonating, you will not suffer any damage. Obtained by stopping the thief from stealing the bomb bag at midnight on the first day while you're in Link's form. Slice the thief with your sword to liberate the bomb bag.
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 * How to Get

Bremen Mask
This mask will make animals follow you around when you use it, and can be used with the guy at the back of Romani Ranch to obtain the most useful mask in the game. Obtained by talking to the music box man by the laundry pool in clock town during the evening on the first or second days.
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 * How to Get

Bunny Hood
Lets Link run faster and jump farther. A very useful mask, especially on long trips. Obtained by going to the Romani Ranch after opening it up or when it opens up on the third day, and visiting the man in the chicken barn at the back of the ranch. Use the Bremen Mask to find all the yellow chicks and make them follow you, and they'll turn into full-fledged chickens. Talk to the man there, and he'll give you the Bunny Hood.
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 * How to Get

Mask of Scents
This will enhance your sense of smell, enabling you to find mushrooms in the forest which you can sell. Obtained after beating the first dungeon, after the swamp is restored the Deku King's butler will send you through a maze the result of which will give you the Mask of Scents.
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 * How to Get

Stone Mask
This mask causes enemies to ignore you completely and is very useful for dealing with Redeads, Gerudo Guards, or other nasties. Required to beat the game. To obtain it you need to have beaten the second dungeon, obtained Epona and the Lens of Truth, and bought a Red Potion. Use Epona to jump over the gate to the east and use the Lens of Truth to find a man sitting in a ring of stones. Give him a Red Potion and you'll get the Stone Mask.
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 * How to Get

Mask of Truth
You use this mask to read Gossip Stones and the minds of animals. This mask can be found by finding all thirty Skulltulla Tokens in the Swamp Spider House.
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Captain's Hat
This mask fools Stalchildren into thinking that you are one of them. Required to beat the game. You receive it when you play the Sonata of Awakening near the large skeleton in Ikana Graveyard, then by killing him.
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 * How to Get

Gibdo Mask
This mask lets you talk to Gibdos. Required to beat the game. Play the Song of Storms at the dried up fountain in Ikana Canyon, then go into the music box house (Make sure the girl doesn't see you), then go inside and play the Song of Healing in the basement when the Gibdo/human appears.
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 * How to Get

Quest-related Masks
These masks generally just serve a single purpose, usually to show to someone to allow you to gain a piece of heart.

Kamaro's Mask
Causes Link to perform a dance when worn. Use it to dance for the two girls who dance in west clock town at night to obtain a piece of heart. During the nighttime, head north of Clock Town and look for a man dancing on top of a large rock where some eerie music plays. Play the Song of Healing for him, and you'll receive Kamaro's Mask.
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 * How to Get

Don Gero's Mask
Let's you talk to frogs. You'll have to use it on four different frogs found throughout the world to obtain a Piece of Heart. This mask must be obtained after beating the first dungeon and obtaining the Goron's Mask, but without ending the winter in the Goron Village by beating the second dungeon. Light all the torches in the goron city with a deku stick, and use the Goron's Spike Roll to break the rotating pots at the very top of the city to obtain a Rock Sirloin. Carry it all the way to the Mountain Village and throw it to the starving Goron to get this froggy mask.
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 * How to Get

Kafei's Mask
Causes different reactions and conversations from certain people while wearing it. Show this mask to Anju to further your quest. Obtained from the Mayor's wife after 10 AM in clock town. You'll be asked to be a detective and track down her missing son, Kafei.
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 * How to Get

All Night Mask
Allows you to stay up during the boring stories the old lady at the inn tells. To get this mask you have to save the bomb shop lady from the burglar at midnight on the first day, and wait around until the third day at midnight, where you can buy it for 500 Rupees. You'll need to deposit a lot of money in the bank before you can get a wallet that big.
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 * How to Get

Keaton Mask
Wear it when chopping up the bushes that run away and you'll get to play a game with a keaton for a piece of heart. It is also part of the Anju/Kafei sidequest. You need to show Anju the Kafei mask early on the first day and visit her at night to obtain a letter for Kafei. Drop the letter in the mailbox and wait for it to be delivered to meet Kafei. Show up by the laundry pool again on the third day to get the Keaton mask from the Curiosity Shop Owner.
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 * How to Get

Postman's Hat
This will let you open up mailboxes for a Piece of Heart the first time and Rupees after that. As part of the Anju and Kafei quest, you'll also get a letter to "Mama". Give it to the mailman on the night of the third day, and wait for him to deliver it to get his hat.
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 * How to Get

Garo's Mask
Makes you look like a mad ninja, and will help you out greatly in the desert to the east. Required to beat the game. After you've beaten the second dungeon and obtained Epona, race the Gorman brothers on Epona during the day. Win and you'll get Garo's Mask.
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Romani's Mask
It'll allow you access to the Milk Bar at night. After you've beaten the second dungeon and obtained Epona, complete the Romani Ranch quest by defending the barn from aliens at the end of the first day, and by riding with Romani's sister to protect the milk from the Gorman brothers.
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 * How to Get

Circus Leader's Mask
Makes repeating the Romani Ranch sidequest easier since they won't attack you while you're wearing it. After going through the first three dungeons and obtaining all of link's forms, head to the Milk Bar and play all your instruments for the Zora there. The Gorman's brother will give you his mask.
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 * How to Get

Couple's Mask
This mask will put an end to the argument in the Mayor's office and get you a Piece of Heart. Complete the entire Anju/Kafei sidequest, after beating the first three dungeons.
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 * How to Get