From Sega Retro
MESS, or Multiple Emulator Super System is an ambitious emulation project largely built on the concepts pioneered by MAME (and the MAME core). The aim is to emulate almost every home video game console and computer ever created, similar how MAME attempts to emulate all arcade systems. This means MESS, will at some point, emulate all Sega consoles - the SG-1000, SC-3000, Sega Master System, Sega Mega Drive, Sega Game Gear, Sega Mega CD, Sega 32X, Sega Saturn and Sega Dreamcast. 479 systems and 1282 variations of these systems are supported, though many are not yet functional.
Like MAME, MESS attempts to emulate systems as accurately as possible, not necessarily in an efficient way. This means it requires higher system specifications than some dedicated emulators, but it is hoped that the MESS project will deliver better results at the end.
Though MESS is a fairly reliable emulator for simpler systems, it is still rendered inferior by many dedicated emulators.