Although this is not strictly emulation, the ability for an Acorn computer to be able to easily read and write to foreign disk formats is very advantageous. This was a particular help historically with emulation, when it was common to transfer cassette or tape images via IBM format disk, say from some Spectrum transfer software to a PC, and then from the PC disk to the Acorn. With the dawn of the Internet, this is now obsolete for this purpose, but is still important as a study of Acorn computer's interaction with other formats.
I'll add the links fairly soon, right now I guess you're more interested in the emulators themselves!
Apple Macintosh
MacFS
AppleFS
MacHFS (PD)
IBM PC
Risc OS 3
MultiFS
PC Access (PD)
Atari
Risc OS 3
BBC
DFS file
Casio Calculator (PD program, possibly on cover disk)