I just downloaded a pretty amazing tool that follows all the great qualifications: free and useful. The tool's purpose is to deactivate heavy vista interface features whenever you're on battery mode. The tool claims to save up to 70% on your battery life. For me, this made my vista OS using on my Dell XPS M1210 go from 3.5 hours of battery life to about 5.5-6 hours!

http://www.codeplex.com/vistabattery