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)
3.
Debian (been around forever....great package management system)
4.
Mandriva (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...

)
6.
Ubuntu (Awesome up-and-coming distro based from Debian geared for the end-user. Uses Gnome as the desktiop environment.)
7.
Kubuntu (Same as Ubuntu, except that KDE is the desktop environment used)
I think that's about all of them.