Game-altering devices are commonly used for finding glitches and unused content. Common cheating devices include GameShark, Action Replay, Game Genie, and Codebreaker.
Memory viewer/editor
A memory viewer displays data (typically in hexadecimal bytes) from a number of areas. For Game Boy/Color games, these include the ROM, VRAM, SRAM, RAM, WRAM and I/O. Writable areas such as the WRAM (used for in-game variables) can be edited to achieve things such as forcing Pokémon to appear. For example, editing the memory address D059 in Pokémon Red and Blue is equivalent to using the GameShark code 01xx59D0; the "instant encounter" with a Pokémon or Trainer code.GameShark
The GameShark is a very commonly used cheating device, owned by Mad Catz.GameShark has also become a verb, i.e. "to shark" meaning to use a GameShark or "sharking", which refers to using a cheating device like the GameShark.
The Game Boy GameShark code format is 'ttvvaaaa'. tt is usually 01. vv is the index number of what you want to change an address to. "aaaa" is a two byte value that represents a memory address with the lower byte first.
Example code: 010259D0 (010259D0). This writes 02 to the address D059.