This was news to me and has come in really handy in switching from my older iBook to a new TiBook. If you start up a PowerBook G4 (or a newer iBook) while holding down the "T" key, your computer will start up in "firewire target disk mode" ("t" standing for "target") It will then behave just like a firewire hard drive. Once you see the firewire logo on screen just connect that mac to any other computer with a standard firewire cable and its hard drive pops up on the other computers 'desktop.
I copied gigabytes of data (my entire Users folder) with ease this way. Also you could do this to run disk diagnostic software and such on the drive. Very handy.
[Editor's note: You should be aware that if you use target disk mode, the computer you connect the laptop to will have full access to everything on the laptop; permissions are not obeyed on the attached computer. Since you'll most likely be the one connecting the machines, this may not be a big concern to you ... but you should be aware of it. Thanks to an anonymous tipster for pointing this out to me.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020401233536141