RiscOS is a commercial product. Please do not ask me where to find ROM images, especially of 3.6/3.7.
Since I have been asked this several times before :
Arculator emulates an Archimedes (anything from an A3xx to an A5000, up to RiscOS 3.1) and is intended for running games.
RPCemu emulates a RiscPC or A7000 (RiscOS 3.5 and greater) and is intended for running applications (+ some games).
You will probably find the Universal Boot Sequence (836k) quite useful, this is the
version from the Acorn FTP site, but without the !ARMovie software. It also
includes the HostFS module.
Or you can download it on a formatted 50mb hardfile (801k).
Hform might be useful as well. This version (17kb) supports hard discs up to 32gb.