Mine was originally the Final Cartridge III, but it felt very buggy and flawed. Then I got Action Replay and I fell in love -- it was everything that the FC III tried to be! I always wished there were an Action Replay for the modern PC (for Windows XP and higher), but never found anything like it. I am going to attempt to code my own software version one day, but never find the time.
Although I am not a PC Games Developer, surely cheats in modern PC games are more complex than cheats used to be in old 8-bit games? Wouldn't this be a very difficult thing to do?