Linux distros for PowerPC Macs (I mention this specifically since Debian has a port for 68K Macs):
1. Fedora Core 4
(recently released and has PPC support now)
2. Yellow Dog Linux
(one of the first successful Linux/ppc distros out there, based from Red Hat, and now Fedora)
(been around forever....great package management system)
(another RPM-based distro, geared to users new to Linux. Formerly called Mandrake Linux.)
5. SuSE Linux / Novell Linux
(don't know what they are called anymore now that Novell is in charge...
(Awesome up-and-coming distro based from Debian geared for the end-user. Uses Gnome as the desktiop environment.)
(Same as Ubuntu, except that KDE is the desktop environment used)
I think that's about all of them.