if you are running Vista then you might appreciate the Vista Battery Saver. As Tamir Khason describes his project:
This tiny program will save up to 70% of your battery by disabling those nice, but greedy Vista features. Running in task bar with private workset of 5.5M and 0% CPU it will do all work for you, by enabling and disabling customizable features when power source changed or battery power fall under certain percent.
Unfortunately, I’ve heard that Vista aggressively drains your battery (especially Aero, though the Vista team disagrees). Hopefully, they’ll start to build more energy-saving functionality into the core operating system.