The definitive link from a Risc OS machine to a Psion Organiser II. It requires the PC Link adapter on the Psion to deal with the other end.
As far as I am aware it is the only freeware software to link the two machines which operates on the desktop.
Regretably this does not work with the 3a, 3c, series 5 or indeed any machine other than the now rather elderly Psion Organiser II (often available very cheaply at you local Cash Converter - I'd recommend going for the later ones with four line displays).
PsionLink was originally a shareware application with enhanced features in version 2 for registered users. Nobody registered it so I have now re-designated version 2 as freeware.