Pokémon FireRed and LeafGreen/Secrets

Get the chance to get the 3 dogs!
In FireRed and LeafGreen you can get the 3 dogs depending on your starter Pokémon. Here are the steps:
 * 1) Beat the Elite Four
 * 2) Complete the Ruby and Sapphire side-quest
 * 3) Randomly in the wild, you can get 1 of the 3 dogs, which ever has the type advantage to your starter Pokémon.

WARNING: There is a glitch in the game and if you let the Pokémon use Roar on you, it will disappear from the area section on the Pokédex and you will never see it again.

Unlockable
How to unlock:
 * Entei in the wild: Your starter must have been Bulbasaur.
 * Raikou in the wild: Your starter must have been Squirtle.
 * Suicune in the wild: Your starter must have been Charmander.

Getting the National Dex (JP version)
Once you have beaten the Elite 4 and have more than 60 Pokémon in the Pokédex, go to Pallet Town and see the Professor. He gives you and Green the National Dex, which allows you to view the entries of all 386 Pokémon.

How to get the Three Unique attacks
You have to have gone to the second island. Your Pokémon have to trust you enough and you have to have the one of three starters.(only the one you started with can learn these moves. (Frenzy Plant, Hydro Cannon and Blast Burn)

Itemfinder Secrets
There are places in the game where important people/Pokémon have stood at one point. If you use the Itemfinder on the EXACT spot where they were, a special item will be found which is usually very rare. You cannot find these items any other way.

How to unlock:
 * Leftovers: Use Itemfinder where the 2 Snorlaxes appeared.
 * Macho Brace: Use Itemfinder where Giovanni was in Viridian Gym.
 * Soothe Bell: Use Itemfinder where Mr. Fuji was standing in Pokémon Tower

Lorelei's Doll Collection
For each 25 times you enter the Hall of Fame, she'll add a new doll to the collection in her house on Four Island. She'll stop after adding a total of eight dolls.

Nugget on Three Island
When you first visit Three Island, you'll come across a miner looking for gold in one of the caves. If you return to him after beating the Elite Four, he'll hand over a Nugget, an item that can be sold at high prices.

The Poké Mart on Two Island
When you first visit Two Island, the local Poké Mart will be severely limited in the items for sale. As you complete various tasks, more items will become available.

How to unlock:
 * Lava Cookie: Complete the Network Machine
 * Lemonade: Obtain the National Pokédex
 * MooMoo Milk: Obtain the National Pokédex
 * Repeat Ball: Complete the Network Machine
 * Soda Pop: Rescue Lostella
 * Timer Ball: Complete the Network Machine
 * Ultra Ball: Rescue Lostella(she is secretly on Mt. Ember)

Trainers: Some pokemon are easily trained

Unlock trading to and from Pokémon Ruby, Sapphire, and Colosseum
At the beginning of LeafGreen/FireRed, you can only trade between the two respective versions. However, if you complete a certain task in the game, you can trade to Pokémon Colosseum on the GameCube, and Pokémon Ruby/Sapphire, the two other Pokémon games on the GBA. Defeat the Elite Four, then retrive both the Ruby from Mt. Ember and the Sapphire from Five Island Rocket Warehouse, then give them both to Celio.

Unlocking mini-games on Two Island
When you have access to Two Island, save the Game Corner owner's daughter, then deliver the package. The grateful owner will open some mini-games that you and your friends can play.

Shiny Pokémon
Every time you encounter a Pokémon you have a 1/8192 chance of the Pokémon being a different color, this is called a shiny Pokémon

Saving Time in the Safari Zone
When you are in the Safari Zone, it counts the number of steps you take to determine the amount of time you have left. Well, if you want to catch a rare Pokémon, but don't want to run out of time, enter the grass, then lightly tap the directional pad. You will change the way you are facing, but it will not count as a step. You can have near-infinite time in the Safari Zone doing this. It is helpful when some Pokémon, such as Chansey, appear in only one section of the Zone. Also if you have a Pokémon that knows sweet scent you can use that without taking time up.

Determining Pokémon's feelings towards you
When you use an item on one of your Pokémon outside of a battle, such as a Potion, the Pokémon will either move closer or away from you in the short intermission sequence. How far the Pokémon moves depends on how much it likes you. If it does not move at all, it feels neutral. You can also take your Pokémon to daisy in Gary's house and ask her.

Ditto Breeding
You don't need to find one Pokémon of each gender in order to breed them. You simply have to put your Pokémon of choice and a ditto into the daycare center. After a short while, the old man will tell you that they found an egg, and when it hatches, it will be whichever Pokémon you matched up with Ditto.

Easy money
On Island 5, there is an area to the northeast called Gorgeous Resort. There are 2 wealthy trainers you can face. They are named Lady Jacki and Lady Gillian. Lady Jacki gives you $10000 if you beat her ($20000 with Amulet Coin) and Lady Gillian gives you $9800 ($19600 with Amulet Coin). The Pokémon they use also carry expensive items. Jacki's 2 Pokémon carry Stardust, Gillian's 2 Mareep have Stardust, and Gillian's Flaafy has a Nugget. Just use Thief or Covet to obtain these items.

Free Coins
In the Gamecorner in Celadon City, go talk to the man at the far right Slots. He will give you 20 free coins. Talk to the man on the slots beside him. He will give you 20 free coins, also. Go to the left side of the Gamecorner. Talk to the guy that looks like a fisherman, and he will give you 10 free coins. There are also hidden coins on the ground so use the item-finder or press on every tile.

Free Nest Balls
There's a woman that lives along the Water Path that's looking for a Heracross. If you catch one and bring it to her, she'll measure it and give you a Nest Ball as a reward. Each time you bring her a taller Heracross, she'll give you another Nest Ball.

Loose change in Game Corner
Apparently, the folks in the Game Corner don't hold on to their coins very well! By using the Itemfinder or looking by yourself, you'll find that many spots in the Game Corner have some number of coins lying on them. If you find them all, you'll find 200+ coins!

Unlimited Nuggets
On the bridge on Route 24, above Cerulean City, there are five trainers that you must beat to pass. At the end, a sixth trainer congratulates you for defeating everyone else by giving you a Nugget. If you lose the following battle against him, you will white out and come to inside the Pokémon Center, as you would any other time, and you will still have the Nugget in your bag. When you go to cross the bridge again, the whole conversation will have reset with the final trainer, and he will give you another Nugget and fight you again. You can lose the battle yet again, and repeat this as many times as you want, and even continue past him by using a "walk through walls" code so that you can come back later to get and sell even more Nuggets for 5000 each.