Let's face it, one of the biggest energy wasters is lighting rooms or closets that no one is using. It is really easy to turn on a light, walk away, and forget it for hours. That's energy and money gone. One way to combat this is by using a motion detector to ensure that you only light a room when someone is in it.
In the forums we've got a new Home Automation HowTo tutorial on setting up a basic system (along with some advanced ideas as well).