Get the National Dex (JP Version)

Once you have beaten the Elite 4 and have more then 60 pokemon in the pokedex, go to Pallet Town and see the Professor. He gives you and Gary the National Dex, which allows you to view the entries of all 386 pokemon.

Contributed by: Starkin Warrior 

Berry Glitch Fix (for Pokemon Ruby/Sapphire)

For this, you'll need two GBA consoles, a link cable, any one of Pokemon Emerald, FireRed, or LeafGreen Versions, and a Pokemon Ruby or Sapphire cartridge. Start up Pokemon Emerald, FireRed, or LeafGreen, and hold B+Select on the title screen. Connect this console to the other containing the Pokemon Ruby/Sapphire cartridge, and follow the directions given to you by the game. This allows you to download a fix to a glitch in Pokemon Ruby/Sapphire that starts near the end of every year and affects time-based events; most prominently, the growing of Berries.

Contributed by: Eevee-Trainer 

Shiny pokemon

every time you encounter a pokemon you have a 1/8192 chance of the pokemon being a different color, this is called a shiny pokemon

Contributed by: Y2JosHBK386 

How to avoid wild Pokemon.

This only works on grass. Once you obtained CUT, you can cut the grass so you won't find any Pokemon.

Contributed by: iambackagain 

Determining Pokemon's feelings towards you.

When you use an item on one of your Pokemon outside of a battle, such as a Potion, the Pokemon will either move closer or away from you in the short intermission sequence. How far the Pokemon moves depends on how much it likes you. If it does not move at all, it feels neutral.

Contributed by: Bendilin 

More time in the Safari Zone

In the safari zone , stand in any grassy area. Instead of moving left and right, just tap left or tap right so that your character turns but you dont move. By doing this, you'll still run into random encounters but your time in the safari zone will not decrease.

Contributed by: Cactus11 

Get to Sabrina in the Saffron City Gym without fighting any trainers

Enter the Saffron City Gym, and step onto the warp panel. Now, go:
North, South, South, North, South, North, North, South, South, North, North, North, South, South. You will now be in Sabrina's room, without having to fight any battles.

Contributed by: Anonymous 

Mystery Gift

Go to any Poke Mart and click on the clipboard near the shopkeeper. The clipboard is a questionaire. Type in the 4 words "Link Together With All" and the shopkeeper should notice and gives you access to the Mystery Gift system. Save the game and restart. At the Front Menu, there should be a new option called "Mystery Gift" With Mystery Gift, you can unlock many things.

Contributed by: myTIMEisNOW 

Diploma

To get the diploma, catch all 386 pokemon (i.e. fill the national dex).

Contributed by: shutdown56 

Secret Trainer Card Stickers

Go to Island Four and go to the lowest house and talk to the man inside and do the following things:

Contributed by: AC Fanatic 

Enable trading to Ruby and Sapphire

Contributed by: saBA 

ItemFinder Secrets

There are places in the game where important people/Pokemon 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.

Contributed by: baconlabs 

Trainer Card upgrades

By completing certain tasks, it is possible to upgrade your trainer card a total of four times, from the intital blue to green, bronze, silver, and gold. The upgrades can be obtained in any order, and each one adds a star to your trainer card and upgrades the color.

Contributed by: GauRocks 

Mystery Gift function

To unlock the Mystery Gift function, you need to go to any Poké-Mart and interact with the white paper on the desk closer to the door; it's a questionnaire and, if you put in "LINK TOGETHER WITH ALL", the clerk will unlock Mystery Gift for you, which you'll from then on (after saving once) be able to access from the home screen.

Contributed by: Andrex_93 

Berry Glitch Fix (for Pokemon Ruby/Sapphire)

For this, you'll need two GBA consoles, a link cable, any one of Pokemon Emerald, FireRed, or LeafGreen Versions, and a Pokemon Ruby or Sapphire cartridge. Start up Pokemon Emerald, FireRed, or LeafGreen, and hold B+Select on the title screen. Connect this console to the other containing the Pokemon Ruby/Sapphire cartridge, and follow the directions given to you by the game. This allows you to download a fix to a glitch in Pokemon Ruby/Sapphire that starts near the end of every year and affects time-based events; most prominently, the growing of Berries.

Contributed by: Anonymous 

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 pokemon they use also carry expensive items. Jacki's 2 pokemon carry Stardust, Gillian's 2 Mareep have Stardust, and Gillian's Flaafy has a Nugget. Just use Thief or Covet to obtain these items.

Contributed by: Crimson1313 

Shiny Pokemon

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

Contributed by: Y2JosHBK386 

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!

Contributed by: baconlabs 

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.

Contributed by: Shop Head 

Determining Pokemon's feelings towards you.

When you use an item on one of your Pokemon outside of a battle, such as a Potion, the Pokemon will either move closer or away from you in the short intermission sequence. How far the Pokemon moves depends on how much it likes you. If it does not move at all, it feels neutral.

Contributed by: Bendilin 

Ditto Breeding

You don't need to find one pokemon of each gender in order to breed them. You simply have to put your pokemon 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 pokemon you matched up with Ditto

Contributed by: squalabaloo 

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.

Contributed by: Destroyer2000 

How to get the Three Unique attacks

You have to have gone to the second island. Your pokemon 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 Blaast Burn)

Contributed by: uniqueskillz517 

Trainer Card Stickers

Go to 'Island 4' and go to the bragget house where he asks you what you want to Brag about.

Contributed by: ShinyCelebi 

Get the chance to get the 3 dogs!

In FireRed, and LeafGreen you can get the 3 dogs depending on your starter pokemon. Here are the steps:
1: Beat the Elite Four
2: Randomly in the wild, you can get 1 of the 3 dogs by your starter pokemon.

Contributed by: JediSage99 

The Pokemart on Two Island

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

Contributed by: discoinferno84 

Itemfinder Secrets

There are places in the game where important people/Pokemon 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.

Contributed by: baconlabs, Mattcraft900 

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.

Contributed by: discoinferno84 

Getting the National Dex (JP Version)

Contributed by: Starkin Warrior 

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.

Contributed by: discoinferno84 

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.

Contributed by: discoinferno84 

Trainer Card upgrades

By completing certain tasks, it is possible to upgrade your trainer card a total of four times, from the intital blue to green, bronze, silver, and gold. The upgrades can be obtained in any order, and each one adds a star to your trainer card and upgrades the color.

Contributed by: GauRocks 

Unlock trading to and from Pokemon 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 Pokemon Colosseum on the Gamecube, and Pokemon Ruby/Sapphire, the two other Pokemon games on the GBA.

Contributed by: discoinferno84