Red/Blue Codes

Mew:
Mew can't be earned in the game. He can ONLY be put on the cartridge
by Nintendo with a special machine. (Or you can use a gameshark/Trade with someone)
One Time Pokemon:
Articuno, Mewtwo, Moltres and Zapdos can only be fought once. If you fail to beat and/or capture them, you won't get another chance. I suggest you save the game before battleing one so if you defeat it you can restart and try to catch it again.
Mewtwo:
Once you beat the game, return to Cerulean City and head for the bridge to the north that held 5 trainers and was on the path to Bill's house. Go left to the water, then SURF into the cave. Mewtwo is inside.
Pokemon Cloning:
Warning: Sometimes *rarely* games may be erased with this code. It's never happened to me before.
Two players trading, player A and player B
A: person with the Pokemon to be cloned
B: person throwing away a useless Pokemon to receive a copy of the original.
* A enters a Game Link center and wishes to clone Mew.
* B enters a Game Link center and wishes to lose Pikachu and get Mew.
* Begin trade. Player B receives Mew.
* B's screen says "Waiting..."
* As soon as B's "Waiting..." disappears, turn off A's Game Boy.
* When B's screen says "Trade completed" turn off B's Game Boy.
* Turn both Game Boys on. Both A and B will have Mew, but the Pikachu has vanished.
Experience Sharing Put a weak Pokemon in position one on your list; when you enter a battle, immediately switch to a strong Pokemon. All Pokemon used in the battle gain experience, even the one that did nothing.
Evolution Tips:
Use the Thunder Stone to evolve Pikachu and Eevee.
Use the Fire Stone to evolve Vulpix, Growlithe, and Eevee.
The Water Stone will work on Poliwhirl, Shellder, Staryu, and Eevee.
Try the Leaf Stone on Gloom, Weepinbell, and Exeggcute.
The Moon Stone will evolve Nidorina, Nidorino, Clefairy, and Jigglypuff.
Machoke, Graveler, Haunter, and Kadabra will evolve only when they are traded via the Game Link cable.
Rechallenge Elite Four:
After you beat the game, the credits role and the game returns to the title screen. Continue your game and you can beat the Elite Four again for even more experience points.
Statue Fishing:
The statues inside the gyms can be used for fishing. Just stand next to a statue and use a rod.
Koga's Gym Walls:
The tiles on the floor of the gym that seem to be outlined with dashed lines represent invisible walls.
Fight Land Pokemon in Water:
First make your way to a location containing the Pokemon you want to battle (such as the Safari Zone; if you go to the Safari Zone, though, let your time run out), then FLY to the water around Cinnabar or Seafoam Island (the squares you are in should be half water and half land). Swim along the shore of the island and you will be attacked by the Pokemon that were at the last land area you visited.
MissingNo.: Trade with anyone in the first room of the Pokemon Research Lab on Cinnabar Island. Exit the Lab, go east, then surf up and down the coast (you should be in blocks that are half water and half land) until you are attacked by a MissingNo. If you catch him, be aware that saving the game afterwards seems to cause many problems... it may even erase your game.
Catch Pokemon at Level 100 and Higher:
Please note that this only works on the Red version of the game. First, talk to the man near the northern edge of Viridian City and watch him catch a Weedle. FLY to Cinnabar Island, then surf along the shore. You will meet up with very high level Pokemon. Please note that if you catch these Pokemon and use them in battle, they will return to level 100 if they earn any experience points. However, they can be used without any problem in the two player battles.
99 Items:
Put the item you want 99 of into the sixth slot of your inventory and FLY to Viridian City. Speak to the man at the north edge of town, tell him you aren't in a hurry, and watch him catch a Weedle. FLY to Fuchshia City, walk south to the edge of the water, then SURF to Seafoam Island. Swim up and down the beach and you will eventually meet a glitch Pokemon named M. Kill him or run away, but don't catch him; after the battle, you will have 99 of the item.