Pokemon: LeafGreen Cheats

Completing the Pokidex
Trade with Pokimon Ruby/Sapphire, Pokimon Colosseum, and Leaf Green to complete the Pokidex.

Unlock Islands 1-3
Beat the first 7 gyms.

How to find the legendary bird pokemon
There are three legendary bird pokemon. Their locations are as followed:

Articuno: found in the seafoam islands, requirements: HMsurf, HMstrength
Zapdos: found in the power plant, requirements: HMsurf
Moltress: found in the cave by cerulean city, requirements: all HM's, beaten the elite four

Lost Cave Paths
1st: Right, Down - Lax Incence
2nd: Right, Up, Down, Up - Sea Incence
3rd: Right, Up, Down, Down, Right, Right - Max Revive
4th: Right, Up, Down, Down, Right, Left, Down, Left - Rare Candy
5th: Right, Up, Down, Down, Right, Left, Down, Right, Up - Selphy (and Silk Scarp on second trip)

Mystery Gift
Go into any Pokemon Center and go to the Clipboard by the Shop Owner. Go to it and you can put in 4 words. Put in the words "Link Together With All." The Shopkeeper will notice this and give you access to the Mystery Gift system. Now save your game and reset and when you go to re-load it will be right below "New Game."