In-browser Acorn Archimedes emulator. It covers a selection of the major machines before the RiscPC arrived on the scene.
How did I not know about this already?!
The Archimedes should be a relatively easy machine to emulate: the ARM3 and earlier have a straightforward ISA, and the MEMC and VIDC chips are the only other complex custom hardware, and are relatively simple for what they manage to do, as the Archimedes was all about raw processor grunt.