Legend of Zelda: Oracle of Seasons
  
Gameboy


  


Noble Sword:


  While you are in the middle of the Lost Woods, you must change the season to 
  winter, go left, change the season to fall, go left, change the season to spring, 
  go left, change the season to summer,go left, and you should find the Noble 
  Sword. This only works in Oracle of Seasons and NOT in Oracle of Ages. To get 
  the sword in Oracle of Ages you must trade for the Old Zora's Broken Sword and 
  get Patch to fix it. 
Master Sword:


  After you've beaten Oracle of Ages and are halfway through a password-linked 
  Oracle of Seasons game. Go through Hero's Cave and talk to the Zora on a raised 
  platform. He'll give you a password to take to King Zora in your saved Oracle 
  of Ages game. He'll reward you with a sword upgrade. If you have the Wooden 
  Sword, you will receive the Noble Sword. If you had the Noble Sword he will 
  give you the Master Sword, the Lv.3 Sword. Afterwards he will give you a password 
  to take to Farore in the Hall of Secrets in Oracle of Seasons (inside the big 
  Maku Tree) to get that same sword upgrade.
Shield Power-up:


  After you get the pirates to take you to the Holodrum graveyard, enter the house 
  just to the left of the ship. Talk to the ghost to get a password. Then switch 
  over to your Oracle of Ages game and tell the password to the man in the back 
  of the Eyeglass Isle Library in the past of Labrynna. He'll give you a shield 
  upgrade and a password to take to Farore in Oracle of Seasons.
Extra Heart:


  Container Right after you talk to the Maku tree for the first time, you'll find 
  a girl near fountain in the middle of Horon Village. She'll give you a password 
  that you can take to the fairies in the Fairies Woods in the present. They'll 
  give you another Heart Container and a password. Take that password to Farore 
  in Oracle of Seasons to get that same Heart Container.
Upgrade From Bombs to Bombcus:


  You'll notice that the man playing the music in the Woods of Winter has a single 
  boy as an audience. He'll give you a password to take to Troy in present-day 
  Labrynna of your saved Oracle of Ages game (he's in the Gorons' mine cart game 
  where you have to shoot the blue crystals with your seed shooter). If you play 
  a perfect round of the game, you'll get the bombchu, a homing bomb that locks 
  onto the enemies. He'll tell you a password, too. (If you haven't looked at 
  the previous codes, then you should take it to Farore in Oracle of Seasons.
Spin Ring:


  The Spin Ring allows you to spin twice when you power up your sword. After you've 
  acquired the power of autumn and gone to Lava Lake, you'll meet a Subrosian 
  that'll give you a password that you can take to Mayor Plen of Lynna City. He'll 
  give you the ring.
Snowshoe Ring:


  The Snowshoe Ring stops you from sliding on ice. Talk to Mamamu Yan's mother 
  in the Floodgate Keeper's House for a password. Take the password to Mamamu 
  Yan. Win the minigame you will enter for the prize.
All The Power Rings:


  The first power ring is in the Explorer's Crypt. The second one requires you 
  to either plant a gasha seed in the desert, or in the middle-of-water area (you'll 
  need either the flippers, or Dimitri), and fight up to 250 enemies. The last, 
  power ring will require the "Gasha" ring (found by Maple). Equip that, and go 
  to an area where unlimited monsters appear (oh yeah, plant the seed in the desert), 
  fight them, for around 10-15 minutes, then leave. Unequip the Gasha ring and 
  go to the Gasha plant, and you'll get it 



  Thanks to Revolution readers zelda05, Xander10031 and Racer X!