Pokémon Red / Blue

Cheats

Battle Safari Zone Pokémon
-Go into the Safari Zone and enter the area that you want to catch Pokémon from later on
-Once your time is up, exit the Safari Zone and then fly over to Cinnibar Island.
-Once there, go over to the eastern (right) side of the island and surf along the edge.  Safari Zone Pokémon from the area you last encountered a Pokémon at will show up as wild Pokémon, with the ability of being able to battle them.

Battle Pokémon from anywhere in just one place
-Use the Safari Zone cheat, but just go anywhere and encounter a Pokémon instead of the Safari Zone.
-NOTE:  You cannot get the Legendary Birds or Mewtwo through this way.

How to get the mysterious M, Missingno, and Pokémon over level 100
-Talk to the man who teaches you how to catch a Pokémon instead of going to the Safari Zone.  You should be able to find M and Missingno, which look like barcodes, and Pokémon over level 100.  Different Pokémon show up in different games, so you never know what you may run into.

Duplicating Items
-Perform the "Finding M and Missingno" cheat above.
-Move whatever item you want duplicated to the 6th spot on your items list.  REMEBMER, IT MUST BE THE 6TH ITEM OR YOU WILL NOT GET THE RIGHT ITEM DUPLICATED!
-Once you encounter and defeat a M or Missingno, the 6th item in your items list will be duplicated to over 100.  With that, you can duplicate several items and get several effects, like permanent stat modifiers to raise your Pokémon's stats, Rare Candies to get Pokémon to level 100 quicker, or Nuggets to get infinite money!

Duplicate Pokémon in a link trade
-Trade a Pokémon as usual, but you must remember one thing.  The Pokémon that you're trading that isn't going to be duplicated should not be an important Pokémon.  Otherwise, you will lose that Pokémon.
-After the trade is complete, shut off your Game Boy, but have the one with the Pokémon to be duplicated Save his game.
-When you turn on your Game Boy, the Pokémon that you traded will still be there, as well as the one that you "traded" to him.