(page updated December 21, 2015)
MESS (Multi Emulator Super System) emulates many consoles and computer systems, including Apple's Macintosh 128k, 512k, Classic, Plus, 512ke and SE models. It is ported to various systems.
This description of the Macintosh emulation included by MESS was posted by 68kmla member Gorgonops, and is illuminating:
“MESS has the same goal as MAME, which is to emulate as accurately as possible *the hardware* of a given computer or video game system. This goal is fundamentally incompatible with the philosophy of emulators such as BasiliskII (and to a slightly lesser extent, vMac) which use “hacks” to simplify the emulator or increase performance.
Essentially what the MESS/MAME people do is reverse engineer the behavior of the integrated circuits which make up computer systems and virtually put them together into complete 'machines'. This makes the MESS/MAME code base a great resource for authors working on their own emulators, since many of the ICs that exist in MAME/MESS are found in machines from multiple manufacturers and MESS/MAME's code might provide better insight in the working of the chips than even the original data sheets might. However, unless a given emulator is similarly built to emulate the 'hardware' of a machine rather than the 'behavior' it's less easy to import a complete emulator back *into* MESS.”
MESS's mac.c emulator assembles a complete virtual Macintosh from a library of IC definitions, some of which work more completely then others, and then boots a completely unmodified ROM. Which means that MESS has to emulate the IWM controller well enough to fool the ROM into thinking it's getting data from real 400/800k disks, it has to emulate enough of the NEC SCSI controller to allow the system to believe it has a real hard drive attached, etc, etc.
Anyway, the end result is that MESS burns a lot more CPU than vMac and is clunkier and far less friendly to use. However it *does* emulate a 'real Macintosh', not simply a 'Macintosh-like environment' which depends on ROM patches. Of course, last I checked MESS still couldn't boot System 7 because of unresolved issues with various individual components, but… someday, perhaps, it will be able to run things that vMac or BasiliskII won't be able to. A/UX on the MacII/SE 30 emulation, for instance. You just can't hold your breath. If there's any one thing true about MESS it's that the 'bigger' the machine the longer it takes for reverse engineering to take place. They've pretty much nailed most 8-bit platforms you can think of, but beyond that things tend to be perpetually 'in progress'. Things 'could' be better… after all, MAME is much further along in supporting 16/32 bit game platforms, but that can largely be attributed to greater developer/public interest in arcade vs. computer emulation. A lot more people want to play 'Metal Slug' than care about running obscure bare-metal OSes from twenty years ago.“
Find the original post at the 68kmla forum. Check there for some discussion of the similarities and differences between MESS, Mini vMac and Basilisk II.