ArcEm – The Acorn Archimedes Emulator
arcem.sourceforge.net
arcem.sourceforge.net
https://www.marutan.net/rpcemu/index.php
If you just want the OS on x86 with minimal hassle there is also a commercial emulator:
http://www.virtualacorn.co.uk/index2.htm
This company now owns the source code for one of the two forks of RISC OS. The other is now fully FOSS and runs natively on the Raspberry Pi:
https://www.riscosopen.org/content/
Here is a good beginner's distro of it:
RISC OS can also be installed on a Raspberry Pi [4].
[1] https://en.wikipedia.org/wiki/RISC_OS
[2] https://en.wikipedia.org/wiki/Acorn_Archimedes
[3] https://en.wikipedia.org/wiki/ARM_architecture_family#ARM2
[4] https://www.riscosopen.org/content/downloads/raspberry-pi
Those outside the RISC OS world may be surprised that in market potential terms RISC OS peaked at 2. By the time 3 came along PCs were displacing the Archimedes almost everywhere, and one consequence of that is a lot of software, especially early games, never made it even to the Risc PC.
On some level I have grown to believe that Acorn were deliberately not producing competitive desktop machines so as to keep ARM off the Intel radar. It has become clear from some leaked documents from Acorn (and discussion on here) that they appreciated the game was up, except for the niche enthusiasts, long before the Risc PC was announced.