You need a PPC (PowerPC) Linux distribution in order to install it on an Apple Macintosh. There are several -
Debian,
YellowDog,
Madrake, [URL=http://www.gentoo.org[/URL]. There are probably others, but those are the ones I could think of off the top of my head.
I installed Debian on a Powerbook G3 once (it was a Wallstreet 292MHz model with 192MB RAM) and it went well. Read the installation documentation carefully though - it's quite different from installing Linux on a regular IBM compatible PC.
If it's a *really* old laptop it's not the PPC release you should get, it's the m68k architecture. Debian has a port for it (as they have for pretty much every architecture out there

).
Håkan