MorphOS (
http://www.morphos-team.net/) is available for all G4 based MacMinis.
It has been announced for EMac and PowerMac-G4 (some details need to be cleared here).
It has been been demoed on some PowerBook, but it has been not confirmed that this version will ever be released.
Porting to even older (and completly obsolote) G3 or slower Macs is not planed.