The Multi Emulator Super System, often referred to as MESS, is a multi-system emulator that aims to accurately emulate multiple systems, with an added side effect being the ability to play these systems. In total, MESS currently is able to emulate 685 unique systems, as well as 1,771 total system variations.
It is a sister project to the Multiple Arcade Machine Emulator, MAME. Many of the contributors are the same, and releases take place at the same time.
For a regularly updated list of systems MESS emulates best and a list of emulators that emulate some systems better check out NonMESS