Anyone with a laptop will know that the more you use the processor, the the shorter your battery life is going to be. I typically use Microsoft Word to take notes in my college classes, and with all the possible settings optimized (i.e spelling and grammar off), it is using roughly 16 percent of my TI 800's CPU. PowerPoint idles around 50%. Enter Open Office. Using the X11 port, the total system CPU usage is under 5% for either the word processor or the PowerPoint-like program with spell checking enabled! If that means a little more battery time when I really need it, goodbye Microsoft Word.
[robg adds: When I really want to just jot down writing thoughts on my PowerBook, and battery life is critical, I set the CPU to reduced power settings, dim the screen, disable AirPort and Bluetooth, quit everything, launch TextEdit, and start typing. It's astounding how much longer the battery can last working in this mode ... but if you'd like a bit more functionality, it looks like OpenOffice is a good alternative, though I'm sure there are others ... the general hint is "reduce CPU usage to maximize battery life."]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031124040121934