julir,

in addition to the links given by Markus, there's a long thread here on the knoppix.net forum pages where some more options for the Macintosh/PowerPC platform are being discussed in detail:

Knoppix on Mac

In particular, there's Gnoppix PPC (a live CD - i.e., boot and test-run it without installing to your HD for a start):

http://www.gnoppix.org/

There's Ubuntu for PowerPC, coming with Gnome as a desktop, and available as a live CD as well as an install CD:

ubuntulinux.org

And last but not least, there's Kubuntu, which is very similar to Ubuntu, except that it uses KDE instead of Gnome as a desktop:

Kubuntu Linux

Regards,

Walter.