These codes don't need a Game Shark or Game Genie or anything like that.
Red text indicates that the code works with the Red Version.
Blue text indicates that the code works with the Blue Version.
Yellow text indicates that the code works with the Yellow Version.
MissingNo./Multiply Item (RB) - The most famous code for Pokémon Red and Blue. WARNING: This code destroys the Pokémon League high scores on the PC. Other than that, everything is temporary. This code can give you around 129 of a certain item, and allows you to battle high level Pokémon. Each game has different Pokémon you can battle.
First, go to the second town (I forget its name, stupid sumthin that starts with a v) , the town with the guy who drinks coffee. In the beginning of the game, he's the one that wouldn't let you pass until you got the Pokédex. Talk to him. When he asks you if you're in a hurry, say "no." He'll show you how to catch a Weedle. After he does, fly to Cinnabar Island. If you want to multiply an item, but that item in the 6th position in your item list now.
Use a Pokémon to surf up and down the right side of the island on the brown shoreline. You should run into some Pokémon around level 140 and 129. When you run into a Pokémon named MissingNo., or M, run away. Then check your items. You'll have weird blocks next to the number you have of the 6th item. This code works great for multiplying Rare Candies and Nuggets and stuff like that.
Fish In Statues (RB) - Walk up to a statue in a gym and get out your fishing rod. You can fish in the statue. It's just like the water around it, no special Pokémon. But it is neat if you're in Brock's Gym or something and you fish in the statue to get a water Pokémon.
