SuSE, Gentoo and Debian also have PPC versions. They're about just the same as their x86 counterparts, except that the PPC port of SuSE Linux is a few versions behind. The only difference you will notice is the performance, which might satisfy your curiosity on how the G4/G5 compares to Intel or AMD processors. But the results might still be biased, even though the OSes are the same, because Linux is optimised for the x86.
There are, however, many small OSes which are PPC specific. And, of course, you have Mac OS X and Amiga OS 4.
[ August 20, 2003: Message edited by: Laukev 7.5 ]