3DO Interactive Multiplayer - Released - 1995
The Game Guru is an interesting cheat device in the sense that there's a lot more things available than your standard Game Genie or Action Replay. Sadly, it could only edit save files...and was for the 3DO.
Commodore Amiga - Homebrew - 1992
If you believe that Gameboy Tetris was the very, very best Tetris then this is the Amiga game for you! Two players can enjoy Tetris as God intended it to be, grey scaled and responsive with simple sounds and no music.