There are a bunch of notable companies related to game hacking which not only worked on Cheat devices like the Game genie or Action Replay but also unlicenced products that allowed homebrew games consoles much to the annoyance of the console manufacturer.
Datel is most famous for their Action Replay line of products but also created homebrew enablers for the Gamecube and Nintendo DS along with tons of unlicensed games and applications for a wide range of home consoles and handhelds.
Datel
For more information about Datel check out this post.
Codemasters (Creator of the Game Genie)
For more information about Codemasters check out this post.
There are a bunch of notable companies related to retro game development, many of which specificaled in creating the Software Development Kits for popular consoles such as the Mega Drive (Cross Products) or Playstation 1 (SN Systems).
Cross Products (Creator of Development Tools)
For more information about Cross Products check out this post.
SN Systems is a company that specialised in creating development tools for game developers in the UK, it created development tools for sega and nintendo consoles but is most famous for their Sony Playstation development tools like PSYQ.
SN Systems (Creator of Development Tools)
For more information about SN Systems check out this post.
Interview by the BBC with DMA Design, excerpted from Working Lunch, originally aired on BBC Two on Thursday, May 16, 1996.