Zelda: Majora's Mask

Stray Fairies and Great Fairy rewards:
There are 61 Stray Fairies in Termina: 1 in Clock Town and 15 in each of Woodfall Temple, Snowhead Temple, Great Bay Temple and Stone Tower Temple.
Bring all the fairies from an area to its Great Fairy Fountain to restore the Great Fairy and get a permanent reward.

Fountain      Reward
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Clock Town    Magic Power (as Deku Link), then the Great Fairy's Mask
Woodfall      Spin Attack
Snowhead      Double Magic Meter
Great Bay     Double Defense
Stone Tower   Great Fairy's Sword
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Note: These are the rewards in the original N64 version. In Majora's Mask 3D (3DS) the Woodfall and Snowhead rewards are swapped.

Clock Town fairy:
Bring the Clock Town Stray Fairy to the fountain in North Clock Town as Deku Link to get Magic Power and the bubble attack (press B to shoot bubbles, hold B for a big one).
After you are Hylian Link again (Song of Healing) and have reset time, bring the fairy to the fountain again to get the Great Fairy's Mask.

Great Fairy's Mask:
Wear the mask inside a temple: every free Stray Fairy in the room flies to you, and the mask warns you when a fairy is close.
It does not open chests or burst bubbles, so do that first.

Deliver the fairies in one cycle:
Stray Fairies you have found but not delivered are lost when you play the Song of Time to return to the Dawn of the First Day, so deliver all 15 from a temple within one three-day cycle.
Rewards you have already received stay with you.