I'm a big fan of
Gentoo and
LFS (Linux from Scratch).
IMO those two distros are the best ones for learning from, but the installs are most definately not for the faint of heart or for people who hate to RTFM. Granted, if you struggle through it, you'll learn a hell of a lot more than you will by just popping in a LiveCD and fooling around with things. Then again, it
will drive you crazy if you're not used to the command prompt (or if you really hate it)
But for starters, you can try Ubuntu or Knoppix, both those LiveCDs are cool. CentOS, Fedora Core, Mandriva, SUSE and Debian are fine as well. If you're not ready to start installing things, there's a huge list of Linux LiveCDs over at
http://www.frozentech.com/content/livecd.php so check those out. There's a ton of them, everything from the Slax KillBill LiveCD to GamesKnoppix and others. Just boot from them and you should be ready to go with minimal reconfiguration.
Oh wait, forgot my obligatory *BSD speech too. Then again, OS X would technically fall under that category.
Oh, and don't forget, you NEED to get the PPC version of the installer/CDs/documentation because it will differ slightly from the x86 stuff. Not many distros out there do have PPC versions, but Debian, Ubuntu and Gentoo do, and there's also a PPC only distro called YellowDogLinux you might want to check out.