[4/24/15: Updated for Jessie stable]
I've been meaning to do this for awhile, so I've finally gotten around to giving you my thoroughly complete guide to installing Linux on your old iBook, Powerbook, Mini, or Power Mac. This will be an epic told in five parts: Pre-Installation, Installing the Base System, Installing the GUI, Configuring Stuff, and Bugs & Quirks. There are a few distributions out there that already do much of the work for you--MintPPC, Lubuntu, and Ubuntu MATE--but I'm going straight to the source and installing Debian. In fact, I'm gonna go for a custom install using Openbox as my window manager for a faster, leaner system, and also because this kind of build-from-the-ground-up approach is a great way to learn Linux. So let's get to it!
For my particular install, I'll be dual-booting Debian and Mac OS 9 on an iBook G3 with 640 MB of RAM. If you want to dual boot OS X, the same basically applies, and although this guide will be laptop-centric, most of the steps here apply to desktops, too.
The first thing to do is backup your data because you'll be reformatting your hard drive in order to repartition it. After you've backed everything up, boot your computer with an OS 9 (or OS X) installation disc by inserting it and pressing the "c" key. Then, find the Utilities folder and run Drive Setup (or Disk Utility if you're booting an OS X disc). Next, select your hard drive from the list and click "Initialize...", then click "Custom Setup..."
How many partitions to create for dual-booting? At least two, but I'm choosing three, one for Debian, one for Mac OS 9, and one as a sharing partition (if you intend to install Debian alone, you can choose one partition). The reason for three? Though your Mac partitions are visible in Linux, your Linux partition is not in OS 9. So I like to have a sharing partition I can access from both systems. Also, OS X's journaled filesystem will only mount as read-only in Linux, and mounting Linux partitions in the OS X Finder can be a pain, so again, you may prefer having a sharing partition easily writable between both systems (Somewhat related, there's this Lifehacker tutorial on symlinking your OS X home folder into your Linux home folder for shared access on the same computer).
Back to Drive Setup, I'm choosing three partitions, then choosing their sizes and filesystems. The first (top) partition on the table must be your Debian partition. Choose the size, then choose "Unallocated" for the filesystem type (I suppose it doesn't matter which type since you'll be deleting that partition in the Debian installer later). Then set the sizes of your OS 9 and sharing partitions and choose HFS Plus for their filesystems. OS X users running Disc Utility will want to choose "Mac OS Extended (Journaled)" for their OS X partition and "Mac OS Extended" for any OS 9 or sharing partitions. Also, if you're triple booting with OS 9 and OS X, put them on separate partitions. If they're on the same one, they'll need the native Mac startup disk chooser to know which to boot, and that will conflict with the Linux bootloader.
Once I was done, I had three partitions, one unallocated, one for OS 9, and one for sharing data between systems. From there, I initialized the disk and saw the two HFS Plus partitions show up on my desktop. Now you can install OS 9 (or OS X). You must install the Mac OS first, before Debian, because Mac system installers don't play well with Linux systems already on disk. Reboot and check that everything works, and now you can proceed to the one step missing from all this--getting the Debian install disc!
So go over to Debian.org and find an ISO to download. There are three branches to choose from: Stable, Testing, and Sid (unstable). Stable gives you rock-solid stability, Testing is a bit more adventurous, and Sid is the bleeding edge, may-make-your-system-fry choice. The Stable branch netinstall ISO can be found on this page, and the Testing installer can be downloaded here (for both, choose the ISO ending in "...powerpc-netinst.iso"). If you have to ask how to install Sid, you probably shouldn't be using Sid.
The only thing left is to burn a disc or make a USB stick. People say to burn at a slow speed since Linux install discs can be fickle, so when in Rome...
One more note, a longstanding bug in the Debian installer messes up Apple driver partitions and will temporarily make your OS 9 partition unbootable (OS 9 only, doesn't affect OS X partitions). There's a simple fix for this, however, that is detailed in "Part V - Bugs & Quirks".
I'll see you in our next episode, "Installing the Base System".
Part II - Installing the Base System
Part III - Installing the GUI
Part IV - Configuring Stuff
Part V - Bugs & Quirks