Passwords in Oracle of Seasons

Holodrum Secret
Received From: Defeat Veran at the end of Oracle of Ages.
Told To: Oracle of Seasons Title Screen.
Purpose: Begin a linked game in Oracle of Seasons.

Ring Secret
Received From: The Red Snake in Ages.
Told To: The Red Snake in Seasons.
Purpose: Transport Rings from one game to another.

Zora Secret
Received From: Zora in the Hero's Cave (reachable after level 3).
Told To: King Zora in Ages.
Purpose: Get the Master Sword.

Library Secret
Received From: Ghost in the House near the Pirate Ship (after you help the pirates).
Told To: The man in the library's back room in Ages.
Purpose: Get the Mirror Shield.

Tingle Secret
Received From: Fairy in the cave next to level 2.
Told To: Tingle in Ages.
Purpose: Seed Upgrade.

Troy Secret
Received From: Kid near the windmill.
Told To: Dr. Troy in the Seedshooter Game.
Purpose: Bombchus.

Symmetry Secret
Received From: Received from the girl in Molly's house (the girl who gives you the shovel).
Told To: One of the girls in the middle house in Symmetry City (either one).
Purpose: Ring Box Upgrade.

Fairy Secret
Received From: Lady near the Horon City fountain.
Told To: Fairy in the Lost Woods in Ages.
Purpose: Heart Container.

Elder Secret
Received From: Red Goron in the Goron Cave.
Told To: Goron Elder in the Goron Baseball Game in Ages.
Purpose: Biggoron Sword.

Tokay Secret
Received From: Moblin near the trashed Great Moblin's Keep.
Told To: Tokay in the Museum.
Purpose: Bomb Upgrade.

Plen Secret
Received From: Subrosian in the Lava Lake (where you get the Bomb Flower.)
Told To: Mayor Plen in Ages.
Purpose: Spin Ring.

Mamamu Secret
Received From: Old Woman in the House where you get the Flood Gate Key.
Told To: Mamamu (the Woman with the Dog) in Ages.
Purpose: Snowshoe Ring.

Hero's Secret
Received From: Defeat Ganon.
Told To: Oracle of Ages Title Screen.
Purpose: Start a "Hero's Game", which is identical to the normal game, only starting with an extra heart.

~ Thanks to PinkMonkey.