Everyone remembers Tetris, the classic nes/gameboy game. Due to legal issues and Nintendo's exclusive rights to "Tetris" on video game consoles, the Sega Genesis (or Megadrive) never saw a "Tetris" game released. The unlicensed version was recently confirmed to exist by none other than "Tetris" creator
Alexey Pajitnov, the creator of tetris.
(right). RacketBoy estimates around 10 copies of "Tetris" for Sega Genesis exist today.