Missingno
First of all, Missingno. is a glitch (well thats what Nintendo says if it is why does it have a name), and can easily erase your game disrupt the graphics or scew up the hall of fame. If you still want to see it anyway, just go to Vermillion City, ask the old man to tell you how to catch pokemon, watch his demonstration, then FLY to Fuschia City. From here swim to the Seafoam Islands and swim up and down the half land half water shore until you run into Missingno. You can also get enemies with very high (like 150) levels here.
Duplicate a Pokemon
This code let's you double a rare Pokémon, but the other Pokémon is deleted. First, have one person select a rare Pokémon or one that is at higher level than yours. Second, have the other
person select an easy one to get like Pidgey or Ratatta. Then have the person the has the rare Pokémon turn their Game Boy off as soon as it says WAITING!. Then have the person with the
common one turn their Game Boy off. If all this was done correctly, you each should have the rare Pokémon, and the common one should be gone. You could even make a team of Mewtwos this way.
Pokeball Gimmick
If you press B+UP when your pokeball hits the pokemon, it will act as one the next level up (for example a pokeball will act as a great ball) this doesn't work for ultra balls though.
Safari Zone Trick
To easily catch Safari Zone Pokemon, just go into the Safari Zone, and walk around in the patch of grass where the type of pokemon you want is. Wait until your time (its not really timed, it just stops after 500 steps) is up and then FLY to Cinnibar Island. Now just SURF up and down the coast on the right side of the island and you will eventually run into the enemy you wanted and since there are no more bait and rocks it should be easy to catch those annoying pokemon like Chansey.
Celadon Slot/Game Corner Tips/Free Coins
Some Slot Machines have greater odds than others. Try using the ones on the left near the two guys. If you see a smiley ^_^ face before you start then its a good one. You can also find coins on the floor, just walk everywhere pressing A repeatedly. Of coarse if you realy wanted that Porygon you could use the 99 item trick to get Nuggets sell them and then buy coins.
Phantom PC
To get the Ghost PC find the Hotel in Celadon City and walk over to the right side where the PC would normally be in a PokeCenter. Walk up and repeatedly press A and you should run into the invisible PC.
Items Found With Itemfinder
These are a few items that you can find with the Itemfinder. None of them are very rare items and you can find them elsewhere in the game. Ive forgotten a few, but none of them are that important.
*Full Restore in Victory Road near the exit
*PP up on Route 17 on the right side of cycling road
*PP up on route 13
*Max Ether on the small island on the way to Victory Road
*Nugget in Route 7 underground path
*Full Restore and X Special in Route 5 underground path
*Ether in the Unknown Dungeon on the first floor
*Rare Candy in Cerulean in the backyard of the house in the top left corner nearest the Unknown Dungeon
*Great Ball in the trash in the kitchen of the S.S.Anne (the chef also says different things when you talk to him!)
Catch Ditto Easily
To catch Ditto easy just use a weak pokemon like Magikarp and then switch after Ditto changes into it.
Fish in a Gym
Just for fun you can fish in one of the statues in a Pokemon Gym.