Duplicate Pokemon Glitch
Even these first generation "Pokemon" games allowed players to use an internal glitch to duplicate their Pokemon, although the process was a bit more complicated for earlier versions. To take advantage of this glitch, you need two Gameboys, two copies of the game and a connector cable. Connect both games and power them on. Start a trade between the two games. On Gameboy 1, offer the Pokemon you wish to duplicate. On Gameboy 2, offer a Pokemon you don't mind losing forever. Just before the "Trade Complete!" message appears on Gameboy 1, turn off the device. If you perform this maneuver correctly, you should see "Trade Complete!" appear on Gameboy 2, and the good Pokemon should appear in your stock. Turn on Gameboy 1 and you should still have the good Pokemon. Because the exact time to turn off Gameboy 1 is unknown, you may have to attempt this procedure several times to take advantage of this glitch.
Almost Infinite Items
This glitch gives you far more than 99 of an item in your stock. The items are not actually infinite, but you would be hard pressed to use all of the items you get. You need Pokemon that grant you the Fly and Surf abilities. Place the item you want to duplicate in the sixth slot. Fly to Viridian City and speak with the old man that stopped you for a chat very early in the game. He's slightly north of where you land. When prompted, tell him that you're not in a hurry, and he will make you watch as he catches a Weedle. Fly to Cinnabar Island and head toward the coast to the right. Use the Surf ability to surf along the shallow coastal waters until you find a Pokemon named either "M" or "Missingno" or a Pokemon that is over level 100. Knock the Pokemon out or run away. Do not catch it, especially if it is M or Missingno, or your game file will become corrupted. Check your items, and the one in the sixth slot will have strange symbols in it if you performed this glitch correctly.
Evolve Without Moon Stones
You can force any Pokemon that needs a Moon Stone to evolve, including Clefairy, Nidorino, Nidorina and Jigglypuff, to evolve without using the Moon Stone. You need a Pokemon that has the Mimic ability in its repertoire of attacks. Place the Pokemon you want to evolve in the lead spot in your party. Walk around to get into random battles and immediately switch the Pokemon you want to evolve for the one with Mimic at the beginning of the battle. Use Mimic -- and nothing else -- until the enemy is defeated. The shared experience points will eventually force the evolution to occur.
Diploma
The Diploma item does not have any added benefits, but if you want to collect all of the items in the game, it is one of the last things to collect. Catch all 150 Pokemon, which will require you to trade with owners of "Pokemon Red Version" because not all Pokemon are in any one version of the game. Speak with Professor Oak once you have all 150 Pokemon, and he will give you the Diploma.
Area-Specific Pokeballs
If you want to catch Pokemon from specific areas but don't have the requisite Pokeballs -- for example, Safari Balls -- go to the zone that has the Pokemon you want. For example, go to the Scyther area in Safari Zone to get a higher chance of encountering a Scyther. Instead of hunting the Pokemon, fly from this area to Cinnabar Island. Go to the right coast, as you did for the duplicate-item glitch, and surf in the shallow water. Go up and down the coast and eventually you will encounter Safari Zone Pokemon. Save your game to keep the glitch from being reset.