Gold & Silver

Copy Pokemon/Items:
First,put the item that you want to clone on the Pokemon that you want you to clone.After you do this save the game.now deposit that Pokemon into your current BOX.Then,switch to another BOX and when it says,"SAVING THE GAME.PLEASE DO NOT TURN OFF POWER!!!",turn the game off as soon the word "POWER" finishes.Then you'll have the Pokemon in your party and also in thae BOX that you deposited him in.That Pokemon we'll also have the item on it so you'll have doubles of your Pokemon and item.

Red & Blue

Extra Stats:

Get a Pokemon and put into battle.Stay in the battle until you're Pokemon runs out of PP for one of its move's that it learned on its own.Then bring it to the Pokecenter and deposit it.Then bring it out in your party and heal it at the Pokecenter.Now check its stats and at least one of its stats will be up by at least one point.You also need Bill's PC for this to work.

Pokemon Stadium 1 & 2

Add Extra Damage:

When you're in a battle,perform an attack that causes direct damage and when the name of the name of the attack comes up,press and hold the A button until you're opponents damage meter stops going down.It should have caused extra damage.This code is extremely hard to do.