My favorite is Juice Defender. There is a free version which is less customizable but I recommend that you try it out first before buying the plus or ultimate version. I currently have the plus which I bought when it was on sale. The other thing I recommend is turning off your WiMax radio whenever you don't need it. Use WiFi instead when you're at home.
Also, if you have a rooted phone, try some of the custom ROMs that will allow you to set CPU and undervolt.