Pokemon Ruby, like every other game in the Pokemon series does not have any cheats that you can use to get ahead, and to catch more / rarer Pokemon. Fortunately, there are a number of Gameshark codes that you can use to do just about anything in the game, such as choosing which wild Pokemon appear (this can even be made to make Mew appear), to unlocking new items.
So for those wondering 'What is the Gameshark? What does it do, and how can I use it? ', Wonder no more. Basically, a Gameshark device is used for modifying video games and unlocking new cheats that otherwise could not be unlocked. For example, with the Pokemon Ruby Gameshark codes, you can do things that would otherwise be impossible in the game. While using a Gameshark may not be for everyone, it definitely has its perks. There is a similar device that does a similar thing, called the Action Replay.
In order to start using Pokemon Ruby Gameshark codes, you must first enter the master code, which basically sets the game up for Gameshark use. This code is:
Some of my favorite codes include: Teaching my Pokemon any HM / TM / Move that I like, the 'walk through walls' code, the 'fast egg hatch' code (which means you can breed Pokemon faster than ever – one of my favorite things to do), and the code to catch a wild Latias (one of the most legendary psychic / dragon type Pokemon of all). A word of warning, however, only use these codes if you are sure that you'll never enter an official Nintendo Pokemon tournament, as all games that have been modified are banned.