Rockbox runs on the following platforms:
Archos Player, Recorder, Ondio
iriver H100, H300
iAudio X5
iPod mini, 3G, 4G, color, Nano, 5G
iriver H10
I have listed these (in
very rough terms) in order of how mature Rockbox is on each platform. So, for example, while there has been no formal release for the iriver H100 and H300 platforms, Rockbox is quite stable on those platforms. It is less stable (though still quite usable) on the iPod platforms. The H10 platform is new and still in relatively early stages of development.
This wiki page has a comparison of Rockbox versus the feature set in the original firmware for Archos, iriver, and Apple platforms:
http://www.rockbox.org/twiki/bin/view/Main/NonArchos?topic=FeatureComparisonIt is somewhat out of date, but it nevertheless provides an overview that may help you select a player.