This page contains information on how to access capabilities of your iBook in ways not intended by Apple. I take no responsability for any damage your hardware might suffer from trying this stuff.
On the other hand: No risk, no fun! If you have any links to iBook hacks not mentioned here, please let me know!
monitor spanning on the dual USB iBook:
Klaus Rutemöller figured out a way to enable monitor
spanning on the latest iBooks with Radeon graphics: link
enabling clamshell mode in OS X:
This one's only for serious hackers: Daniel Hazelbaker posted a patch to the Darwin kernel sources that implements a sysctl to disable the automatic sleep on closing the lid. You can find the original thread not here, it's gone and the patch here.
external monitors using Linux:
Using the latest kernel and the m3mirror tool from Benjamin Herrenschmidt (link), you can use external monitors with resolutions > 1024x768 on a Rage128 equipped iBook. However, there seems to be a syncing problem with my late 2001 iBook, I get a wobbly image. If you don't run the power management, you also get clamshell mode working with Linux, btw.