The Acorn Archimedes is a family of personal computers designed by Acorn Computers Ltd in Cambridge, England and sold in the late-1980s to mid-1990s, their first general purpose home computer based on their own ARM architecture (then the CPU and architecture was known as Acorn RISC Machine, or ARM, that later became one of the most widely used CPU architectures in the world, e.g. used in most smartphones. The first Archimedes was launched in 1987.
ARM's RISC design – 32-bit CPU (26-bit addressing), running at 8 MHz, was stated as running at 4.5+ MIPS, which provided a significant upgrade from 8-bit home computers, such as Acorn's previous ones. A claim of 'Fastest' micro in the world and 18 MIPS, was also made during tests.
The models in the family either omitted the Acorn or Archimedes part of the name, with the first models named "BBC Archimedes", while the name, Acorn Archimedes, is commonly used to describe any of Acorn's contemporary designs based on the same architecture. While the computers are no longer sold, computers such as the Raspberry Pi, can still run its operating system, RISC OS (at least later versions) as they use ARM chips that are (mostly) compatible.
ArcEm is a portable open-source emulator for the Acorn Archimedes. There is more info on the homepage and You can download it from Sourceforge.
RPCEmu allows you to emulate classic Acorn systems, such as the Risc PC and A7000. RPCEmu should be considered Alpha Quality code. It has many known and unknown bugs, and all files used with it should be well backed up before using them with RPCEmu. More info on the homepage and download of the mac port is here.