Pokemon Leaf Green Version

GBA

Turn a Poke ball into a Master Ball:
When in a pokemon battle, go to the bag and click Pokeball (plain pokeballs only!). Then when the pokeball is about to fly hold A and B together. Next when the ball is about to open hold select and B and the pokeball should become a Master Ball...(In that it catches pokemon 100% of the time!)

Get Mew:
Beat the Elite Four twelve times.

National Dex:
With 60 pokemon in your pokedex, beat the Elite Four and then go visit the Professor in Pallet Town.

Mystery Gift:
Go to a Pokemart and select the survey. Enter "Link Together With All" then save the game. Your gift will be accessible in the main menu.

Get MewTwo:
Beat Pokemon League and help the person on one island in the Pokecenter. You can find MewTwo in the Ceruleum Cave.

Thanks to Revolution readers Tarius G, Alan Kowy, and Ryan Phillips!