MorphOS
The Apple PowerMac G5 is capable of running MorphOS
Quick and unofficial MorphOS & Mac OS X dual boot guide for Apple Mac Mini PowerPC.
Prepare Amiga HD with MorphOS
A-EON blog post containing info about MorphOS PowerMac G5 1047 port (Trevor’s Amiga Blog)
AmigaGuiden (NO) / AmigaMAD (EN) with test of MorphOS, AmigaOS 4.1 and AROS (issue 60)
OS X & updates
Mac OS X 10.5.4
Lowend Mac: Mac OS X 10.5 Leopard