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.
 * Properties

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.
 * Properties

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.
 * Properties

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.
 * Properties

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 game is beat.
 * Properties

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: 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. 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.

Blast Mask: 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. This mask will allow you to explode at will, coming in very handy if you run out of bombs. You can even raise your shield before using the mask to keep it from damaging you at all.

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

Bunny Hood: This is by far the most useful mask to have around, and the one I usually have equipped when going from place to place in Termina. It lets you run faster and jump farther, and comes in handy in snagging a piece of heart from the Postman's minigame. It's 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, making Link a significantly faster and quicker warrior.

Mask of Scents: 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. This will enhance your sense of smell, enabling you to find mushrooms in the forest which you can sell, as well as some in the inn.

Stone Mask: This mask causes enemies to ignore you completely and is very useful for dealing with Redeads, Gerudo Guards, or other nasties. 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, allowing you to become practically invisibility to baddies. Required to beat the game.

Mask of Truth: This mask can be found by finding all thirty Skulltulla Tokens in the Swamp Spider House. You use this mask to read Gossip Stones and the minds of animals.

Captain's Hat: This mask fools Stalchildren into thinking that you are one of them. You receive it when you play the Sonata of Awakening near the large skeleton in Ikana Graveyard, then by killing him. Required to beat the game.

Gibdo Mask: This mask lets you talk to Gibdos. 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. Then, watch the most powerful moment in the game. Required to beat the game.

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: 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 a mask which gives you some mad dance moves. Use it for the two girls who dance in west clock town at night to obtain a piece of heart.

Don Gero's Mask: 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. You'll have to use it on four different frogs found throughout the world to obtain a Piece of Heart.

Kafei's Mask: 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. Show this mask to Anju to further your quest.

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.

Keaton Mask: 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. 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.

Postman's Hat: As part of the previous 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. This will let you open up mailboxes for a Piece of Heart the first time and Rupees after that.

Garo's Mask: After you've beaten the second dungeon and obtained Epona, race the Gorman brothers on Epona during the day. You'll gain a mask that gives makes you look like a mad ninja, and will help you out greatly in the desert to the east. Required to beat the game.

Romani Mask: 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. It'll allow you access to the Milk Bar at night.

Circus Leader Mask: 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, which will make repeating the Romani Ranch sidequest easier since they won't attack you while you're wearing it.

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