Improve Android battery life by lowering CPU freq
There is a program called SetCPU which will allow you to over/under clock the processor.  The advantage here is the ability to set profiles to have the phone determine when to adjust the processor, such as when the phone is at a certain temprature, or if it's plugged in or not.

I've been using it on my Droid Eris for a while now, and the results have been great.