Deku Mask
Yes, we all know about this mask, a mask that is the head of a Deku Scrub.  This is one of the three main masks in the game, and being one of those, it has the power to transform our hero into the actual form of the scrub, you can now hide in the ground.  Along with that, this mask also comes along with an instrument, a batch of horns. This mask plays a vital role in the game, probably the first you will receive.

Location (At the beginning) 

Goron Mask
Everyone remembers the lovable gorons, so this mask may be very fun to fool around with.  In Majora's Mask the goron race has been frozen over (probably because the moon is fricken' two feet away from Hyrule!) so you would imagine how vital this mask is.  Since this is one of the main three masks, it transforms Link into a Goron, and equips him with Bongo Drums, he can now roll like a boulder to travel at fast speeds through Talmina.

Location (At snowhead after you played the song of healing to the goron ghost in snowhead) 

Zora Mask
The peaceful tribe of water based creatures are returning for another great adventure.  The Zoras are fish like people who live their lives not knowing much about the world above.  Yes, you guessed it, this is also the last out of the three main masks in the game.  You can be transformed into a Zora and be able to swim through the ocean.

Location (push mikau to shore and play the song of healing for him in great bay.
 

The Fierce Deity's Mask
The Fierce Deity is a mega powered adult link, you must wear his mask to transform into him. The catch is, you can only transform into him at boss battles. The only way to get this mask is if you obtain all the other masks first. Sounds unfair doesn't it? Well the answer is no, it is perfectly fair once you realize who you are dealing with. He is about three times the size as Young Link, he carries a different magical sword that does twice the damage as it's proceeded. The Fierce Deity also has much better armor and protection, and most important of all: he looks really cool. It isn't required to get this mask, but, it is a heck of a lot easier with it.  But if you really want a challenge, fight bosses the old fashion way.

He isn't just an improved version of Adult Link either. He has silver hair, completely white eyes, and is equip with a GIANT double edged sword. And when we say giant, we mean the size of Link himself. Also, (this might excite our fans) when you have full hearts, his sword can shoot a beam across the room like in the previous games.

Location the moon. You need to get all the other masks, beat all the dungeons to get this mask.

Giant Mask
You obtain this mask deep within the Stone Tower, before the boss.  This is the last mask that somewhat transforms Link.  When you wear this mask, you become giant.  However, you can only use this mask to transform during the boss fight.  This mask must be used in order to defeat Twin-Moldu, the creature is huge so you must be big to fight it.  But when you wear this mask, it takes up magic power, so be conservative.  Visit the boss page for more info on Twin-Moldu.

Location is in the Stone Tower after you beat the Eyegore.

Captain's Hap
Play the Sonta of Awakening in the Ikana Graveyard.  You can use this map to give orders to stalchilds so they will obey you.  But do not let them see you without this mask on.

Location is in a chest after you beat the captain of skeletons in a race 

Bremen Mask
This mask makes you appear to be the leader of all animals.  With this mask on, you have the ability to play a melody so that animals will follow you.  Screwy ain't it?

Location at night at the laundry pool by the old windmill guy in OOT.

Romani's Mask
You receive this mask from Cremia after escorting her safely to ClockTown.  When you wear this mask, you can enter the Milk Latte Bar.

Location at the Romani Ranch. Beat the aliens who are gonna abduct the Romani Ranch and help Cremia to successfully carry the milk to clocktown. 

The Blast Mask
This is a rather odd mask.  This mask can be found by saving the bomb shop keeper's mother from a theif.  But this mask can make an explosion, it is best to protect yourself with the R button.

Location North Clock town 1st night save the old lady from sakon the thief.

Don Gero's Mask
Use your Goron special roll to break the chandelier in the Goron Shrine and receive the sirloin rock.  Give it to the starving goron in the Mountain Village and he will give you this mask.  This allows you to talk to frogs.

Location in Goron shrine. Lit all the torches on fire and get the sirloin rock to the hungry goron outside.

Garo's Mask
You can receive this mask from the Gorman brothers after beating them in a horse race.  This mask allows you to talk to Garo, the master of ghosts.

Location at Gorman Brothers race track.

Gibdo Mask
After restoring water to valley, sneak into the music shop in Ikana Canyon.  Go to the basement and play the song of healing to Pamela's dad.  With this mask on, you can talk to Gibdo, the great and powerful mummy.

Location near the stone tower (magic house) after you drained the water, go inside the musical house and play the song of healing for a half human, half zombie person. 

Great Fairy Mask
You get this mask from the Great Fairy at ClockTown after finding two fairy fragments.  This mask will attract lost fairies to you while you wear it.

Location get the mask when you collect the first fairy in the game. 

Circus Leaders Mask
You get this mask in the Milk Bar Latte after helping Toto with the sound check.  If you wear this mask in the Cremia Wagon event, the gorman bros. wont attack you.

play songs for Gorman to win this mask in the Milk Latte. (Requirements, have the Romani Mask).

Kamalo's Mask

You can obtain this from Kamaro, the ghost dancer at night in Termina Field after playing the Son of Healing to him.  With this mask on, Link can dance to help save the Rosa sisters in the Gorman Entertainment Company.

Location is on a strange shaped mushroom rock. A girl dancing. 

Kafai Mask
You receive this mask from Mrs. Aroma Doctour, Kafai's Mother, after promising you will find him for her.  You can use this mask to talk to Anju about him Kafai.

All-Night Mask
You can buy this mask from the Curiosity Shop on the third day after saving the old women on the first night.  You can use this mask to get a piece of heart from Anju's Grandma by staying up "all night" listening to her stories.

The Mask of Scents
You can get this mask from the minister of the Deku tribe after beating him in a race.  With this mask, you can sniff out where a mushroom is in the forest to make medicine.

Get this from the butler near the 1st dungeon. 

Postman's Hat
You can get this mask from the Postman in front of the Milk Bar on the third day after mailing a letter to Madame Aroma.  If you wear it, he will give you a heart piece.

Get this when the postman flee the place at the final light.

Stone Mask
Looks much like the spooky mask, but it is far from it.  When you wear this mask, creatures wont realize you exist.  You can get this from Shiro after spotting him with the Lens of Truth near Ekhana Valley.

Near Stone tower. Normal units cannot be able to catch you.

The Couple Mask 
You can obtain this mask from Anju and Kafai after they get back together.  Anyway, this mask will help The Mayer get through his meeting nicely.  In return, he will give you a piece of heart.

Bunny Mask
This mask is one of the returning ones.  But it will not be used the same way in this game. This will be used to make you run a lot faster in some events

Keaton Mask
Once again this mask is here, but there probably wont be a guard who wants to impress his child in this new universe.  You can use this to find the Legendary Keaton Fox!  He will ask you a bunch of quizzes.