Pokemon Emerald Cheats

Johto starters
When you complete the full 202 Pokemon of Hoenn, Professor Birch will call you on your Pokenav and say that he wants to give you a surprise. You can now choose one of the three Johto starters (Chikorita, Cyndaquil, or Totodile).

Magma Symbol
To get the Magma Symbol (which is required to get to the Team Magma Base), you must go to Mt. Pyre after you defeat Team Aqua in the cove next to Lilycove City.

Mew In Emerald
You can find Mew at World Edge's Island. It will play hide and seek with you. After you found Mew, he will attack you. He will be at Level 30 so be prepared.

National Dex
Immediately after you defeat the Elite Four, you will go to Professor Birch. He will give the National Dex to you.

Rare new attack for Pichu
Pichu may learn an extremely powerful attack after meeting certain criteria. The attack is called "Volt Tackle". It has a Base Attack Power of 120, accuracy of 100, and 15 PP. The only negative side is that the user takes 1/3 of the given damage.In order to get Volt Tackle on your Pichu, you first need one Female Pikachu holding the Light Ball item. You may then use one of the following to breed with Pikachu: Ditto, Male Raichu, or Male Pikachu. No other Pokemon will work.After finishing the breeding process, hatch the Egg, and inside will be your lovely little Pichu with the Volt Tackle attack! This may be transferred to Pikachu or Raichu if you wish.

Safari Zone
The Safari Zone is the same as the one in Pokemon Ruby and Sapphire. The only difference is that after you defeat the Elite Four, two more areas appear full of Johto Pokemon that you could not get in any other games (besides Pokemon Colosseum).