[12/13/14: Updated to include information on Debian Jessie]
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, Ubuntu, and Debian's default desktop install. But I thought I'd put up a guide for a more custom install, in this case using Openbox as window manager, for an ultra-fast low memory system and for those of you who like to roll your own. But even if you're planning to go with Debian's default desktop or MintPPC, you should still find the info here useful and hopefully it'll save you some serious hair pulling. So let's get to it!
For this 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 you want 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..."
Now, how many partitions do you want? At least two, but I'm choosing three, one for Debian, one for Mac OS 9, and one as a sharing partition (if you only 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 shared partition that 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 shared 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 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. Then set the sizes of your OS 9 and shared 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 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 can conflict with the Linux bootloader.
Once I was done, I had three volumes, 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 is generally for newbies and servers, Testing is for somewhat experienced desktop users, 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 the disc. People say to burn it 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