Rockbox is a third party group of developers currently working on alternate firmware for several players, including the iRiver H1xx series and H3xx series. So far, Rockbox's progress on the H3xx is limited and there is no downloadable version.
Back to H3xx Series
EDIT: Rockbox have released Rockbox for developers only at the moment. They have got music playback and some people have Rockboy - game boy games on the iriver - working.