X-MESS is the X11 port of the MESS game console emulator.

MESS is virtual machine emulator; it includes emulators for the CPUs,
sound and video chips used in game consoles, as well as special purpose
PALs and detailed information about the memory layout of each machine.

MESS currently emulates the Nintendo Entertainment System (NES), Sega
Genesis/Megadrive, TRS-80 models 1 and 3, Colour Genie, Colecovision,
Vectrex, Apple ][ series, Atari 800, Atari 5200, Sega Master System/Game
Gear, Bally Astrocade, Kaypro 2x and PDP-1.

To play games in X-MESS you must have a copy of the original game,
extracted from a cartridge, floppy or tape image.