Specifically for the computers running the RISC OS operating system (made by Acorn Computers PLC), here is a software I wrote in a past life...

Source code is provided (both in C and ARM assembly).
http://www.filebase.org.uk/software/emulators/508

Remarks are welcome on this thread.
Best regards,
Benoît

Sounds good to me!

I notice some RISC OS versions seem to be unsupported - have you any idea if it runs on the Raspberry Pi flavour of RISC OS? (There's one regularly updated version linked from this tutorial)

Cheers
Ed

Not currently... I think a bit of work is needed at first to make it "32bits" compatible, and only then to make it run on ARMv5,6,7 CPU/memory architectures. There was an emulation layer which allowed 26bits applications to run on 32bits native hardware (named Aemulator, which I do not know the current business/development status).
As I received the RPi recently, then I might work on it, once a suitable RISC OS build being installed on it and my DDE updated as well.
The snapshot provided here is just to show that the emulator can work from a window within the RISC OS desktop (by a using the "Smalltask" virtual screen utility) but I originally designed it to be run in full screen mode.
Source code is provided (mix of C and assembly) with manageable sizes for the less than 10 files included (assembly is heavily macro based).

So briefly stated, I would advice to stay tuned but not to hold your breath .
Regards,
Benoît

Interesting: not only is the author working on an Rpi port of Aemulor, but also the beageboard version is zero-cost. We can hope!
http://buyit.spellings.net/products.php?cat=2