The Amiga's was an OperatingSystem well before its time. Pre-emptive multitasking in 1984, imagine that! It only took the MacOs and MicrosoftWindows about a decade to catch up. As well as this it had a fast, responsive GUI and a CommandLineInterface with commands whose names made sense. Yes, I'm a Unix guy, but saying "grep" when you mean "search" is a pathological violation of the PrincipleOfLeastAstonishment.
Of course it also had no memory protection (one misbehaved app could crash the whole OS), but the speed advantage you got from being able to pass messages right into another app's address space made this a feature, rather than a bug, on a 7Mhz M68000 processor.
See (or possibly merge with) AmigaComputer.
There are many emulators for the Amiga for Mac, Windows, and Linux. Many of the people using them hang out on http://www.amiga.org/
Although the original AmigaOS 3.x ran on a 68k processor, and essentially died in 1994 when Commodore went under, there are now a number of clones being produced:
There is also the AmigaSdk, an Amiga-like programming environment for Linux, Windows, and handhelds.
s As to the lack of memory protection in the Amiga OS - yes it is a problem <- Not True, well written software for the Amiga never has and never will require "memory protection", if the software is written to the standards expected of the OS and is system friendly then "Memory Protection" is meaningless as the properly written software on the Amiga using it built in memory handling structures and routines means that it already has it's own "memory protection".
... but surprisingly many Amiga users report they find it far more stable than a Windows system (especially prior to Windows XP). The lack of protection simply forces programmers to be more careful in testing, and the well-designed OS does not itself cause crashes (where-as Windows crashes are often due to kernel bugs).
The design of the OS was amazingly advanced for the 1985-1994 era it occupied, so that it is still quite compelling even today (hence the number of clone OSes), but people also ought to consider alternatives - such as BeOs, which is purportedly a "spiritual successor" to the Amiga OS as far as design goes.
IIRC the OS was designed by people from a mainframe culture, so multi-tasking was considered fundamental.
It's worth pointing out that the 68000 did not have the capability for memory protection. This was a hardware issue; the OS didn't have any choice.
The 68010 had limited buggy ability in that direction, and required an extra expensive support chip even to do it badly (Sun worked around this with an extremely sophisticated and expensive hardware hack). It wasn't until the 68020 that these processors had the full ability to do memory protection in an uncrippled way, and not only were those not even available when the Amiga was introduced, they were also too expensive, initially, for a product like the Amiga, when they were introduced a few years later.
Of course, once they were available, and at a more reasonable cost, then backwards-compatibility was an issue, as always. Same thing happened to the Mac, and it didn't truly get fixed 100% until they changed both the processor family and the OS (to Mach). This is not true. The '020-using Mac II defined "Mac compatibility" for almost a decade.
Similarly with Windows; it took an OS swap (to NT) to fix the same problem. -- DougMerritt